Pencil lead and production process for the same

ABSTRACT

A pencil lead containing flake graphite which has an ab face having a flatness of at least 2 μm or less and in which an aspect ratio of an a axis or a b axis in the ab face to a c axis is 5 or more is provided with a structure in which nanoparticles having a my value of 0.05 to 2 based on 100 of a volume average diameter (my value) of the graphite and having a sphericity of 0.1 to 20 nm or nanoparticles having a my value of 0.05 to 2 based on 100 of a volume average diameter (my value) of the graphite and having a specific surface area of 200 to 600 m 2 /g are brought into contact with the ab face of the graphite or a structure in which they are adhered to the ab face of the graphite.

TECHNICAL FIELD

The present invention relates to a pencil lead such as pencil leads formechanical pencils, pencil leads for wood case pencils and the like,more specifically to a pencil lead which has high strength and smoothwriting feeling and which provides drawn lines with a deep and vividblack color and a production process for the same.

BACKGROUND ART

In general, important characteristics required to pencil leads are goodwriting feeling, a good color developing property of the drawn lines andhigh mechanical strength.

In order to provide a solid drawing material which is increased in anoil-impregnable effective pore volume and a surface area in a soliddrawing material such as a pencil lead and further enhanced incompressive strength and has smooth writing feeling, a satisfactorycolor developing property and satisfactory drawn line intensity andwhich has a small abrasion amount and a good erasing property and isless liable to be stained when rubbing the drawn lines with a finger anda production process for the same, the present applicant proposes asolid drawing material prepared by forming a solid drawing material leadbody by subjecting a blend composition for a solid drawing materialcontaining at least a nanomaterial (nanoparticles) to burning treatmentor non-burning treatment and filling pores of the above solid drawingmaterial lead body with a lubricant (refer to, for example, patentdocument 1).

Further, it is known as well that a production process for a lead bodyprepared by adhering in advance a part or all of fine particles havingan average particle diameter of 100 nm or less to a surface of a tabularextender to prepare a fine particle-adhered tabular extender, mixing andkneading it with lead body materials and then molding the mixture byextrusion molding is provided for the purpose of providing a process forproducing a pencil lead which is an extrusion-molded lead body capableof forming a thick film containing fine particles in a highconcentration by writing (refer to, for example, patent document 2). Theabove technique includes the content disclosed in patent document 1described above. A thickness of the film described in the above patentdocument 2 is obtained by expressing merely power for masking the groundby a numerical value, and it is not related to the visual color(intensity) and the writing feeling.

On the other hand, the following defects are present in the evaluationitem called “touch in writing” or “writing feeling” in patent document 1described above. That is to evaluate it based on feeling obtained when atester draws lines by a one-sidedly abraded face for a short time whilemaintaining a posture taken in starting the test without almost changinga writing face of the mechanical pencil. The one-sidedly abraded face isa face which is abraded and flat, and therefore the lines are written bya face which is almost abraded and flat until finishing writing sinceinitiating it.

Accordingly, it has been found that when a lead to be tested is appliedto a product developed by the present applicant which is put recently onthe market and gains public favor (a mechanical pencil, trade name:“KURUTOGA”, manufactured by Mitsubishi Pencil Co., Ltd., internationalapplication WO2007/142135 (U.S. Pat. No. 4,240,417)), to be specific,when the test is carried out by applying it to a mechanical pencil of atype in which a lead is rotated whenever written to always carry outwriting by a fresh part, the problem that such writing feeling that hasso far been obtained is not reproduced is brought about.

That is, even if nanoparticles are simply mixed by the art described inpatent document 1 and the like described above to form solid drawingmaterials, favorable evaluations of excellent drawn line intensity,writing feeling in actual writing and a static•dynamic frictionalcoefficient which is a typical index thereof have not been able to beobtained. When measuring a static•dynamic frictional coefficient for asolid drawing material obtained by simply mixing nanoparticles, theproblem that the evaluation item called “the touch in writing” or “thewriting feeling” described above is not always reproduced depending on aproduction process and a constitution of the lead body has been found.

As described above, the existing state is that pencil leads which havebetter and smoother writing feeling and high strength and which providedrawn lines with a deep and brilliant black color and a productionprocess for the same are desired earnestly for pencil leads prepared byusing nanomaterials (nanoparticles) and pencil leads used for amechanical pencil of a type in which a lead is rotated whenever writtento always carry out writing by a fresh part in addition to pencil leadsused for conventional mechanical pencils, wood case pencils and thelike.

-   Patent document 1: Japanese Patent Application Laid-Open No.    2007-138031 (claims, examples and others)-   Patent document 2: Japanese Patent Application Laid-Open No.    2008-115211 (claims, examples and others)

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention

In light of the problems on the conventional art and the existing statedescribed above, the present invention solves them, and an object of thepresent invention is to provide a pencil lead which has better andsmoother writing feeling and which provides drawn lines with highintensity and a brilliant black color and a production process for thesame, wherein it is a pencil lead prepared by using nanoparticles andused for a mechanical pencil of a type in which a lead is rotatedwhenever written to always carry out writing by a fresh part in additionto pencil leads used for conventional mechanical pencils, wood casepencils and the like.

Means for Solving the Problems

Intense researches repeated by the present inventors in light of theconventional problems described above have resulted in successfullyobtaining a pencil lead having drawn line intensity, writing feeling anda low static•dynamic frictional coefficient which exceed those of thepencil leads disclosed in patent document 1 and the like each describedabove and a production process for the same by forming a lead body of apencil lead from graphite and the like, then evenly dispersingnanoparticles having a specific particle diameter and a specificsphericity in a specific liquid or evenly dispersing nanoparticleshaving a specific particle diameter and a specific surface area in aspecific liquid and impregnating the lead body with the dispersionliquid to produce a pencil lead, and thus the present invention has cometo be completed.

Also, intense researches repeated by the present inventors in light ofthe conventional problems described above have resulted in successfullyobtaining a pencil lead having drawn line intensity, writing feeling anda low static•dynamic frictional coefficient which exceed those of thepencil leads disclosed in patent document 1 and the like each describedabove and a production process for the same by bringing flake graphitehaving specific physical properties into contact with nanoparticleshaving specific physical properties, then fixing the nanoparticlesthereon to prepare a composite, forming a lead body by using thecomposite graphite and then impregnating the lead body with a liquidhaving specific physical properties to produce a pencil lead, and thusthe present invention has come to be completed.

Further, intense researches repeated by the present inventors in lightof the conventional problems described above have resulted insuccessfully obtaining a production process for a pencil lead havingdrawn line intensity, writing feeling and a low static•dynamicfrictional coefficient which exceed those of the pencil leads disclosedin patent document 1 and the like each described above by using flakegraphite having specific physical properties, kneading the flakegraphite with nanoparticles having specific physical properties whichare dispersed in a plasticizer or a solvent and then forming a lead bodyfrom the kneaded matter, and thus the present invention has come to becompleted.

That is, the present invention comprises the following items (1) to(21).

(1) A pencil lead containing flake graphite which has an ab face havinga flatness of at least 2 μm or less and in which an aspect ratio of an aaxis or a b axis in the ab face to a c axis is 5 or more, whereinnanoparticles having a my value of 0.05 to 2 based on 100 of a volumeaverage diameter (mv value) of the graphite and having a sphericity of0.1 to 20 nm are brought into contact with the ab face of the graphite.(2) A pencil lead containing flake graphite which has an ab face havinga flatness of at least 2 μm or less and in which an aspect ratio of an aaxis or a b axis in the ab face to a c axis is 5 or more, whereinnanoparticles having a my value of 0.05 to 2 based on 100 of a volumeaverage diameter (mv value) of the graphite and having a specificsurface area of 50 to 800 m²/g are brought into contact with the ab faceof the graphite.(3) A pencil lead containing flake graphite which has an ab face havinga flatness of at least 2 μm or less and in which an aspect ratio of an aaxis or a b axis in the ab face to a c axis is 5 or more, whereinnanoparticles having a my value of 0.05 to 2 based on 100 of a volumeaverage diameter (my value) of the graphite and having a sphericity of0.1 to 20 nm are adhered to the ab face of the graphite.(4) A pencil lead containing flake graphite which has an ab face havinga flatness of at least 2 μm or less and in which an aspect ratio of an aaxis or a b axis in the ab face to a c axis is 5 or more, whereinnanoparticles having a my value of 0.05 to 2 based on 100 of a volumeaverage diameter (mv value) of the graphite and having a specificsurface area of 50 to 800 m²/g are adhered to the ab face of thegraphite.(5) The pencil lead as described in any one of the above items (1) to(4), wherein the nanoparticles used for the pencil lead are carbonnanoparticles.(6) The pencil lead as described in the above item (5), wherein thecarbon nanoparticles are diamond.(7) The pencil lead as described in any one of the above items (1) to(6), wherein the nanoparticles have a volume average diameter (mv value)of 4 to 100 nm.(8) The pencil lead as described in any one of the above items (1) to(7), wherein a total frictional coefficient obtained by dividing anaverage value (n=10) of total frictional forces in lines drawn by adrawing method using a drawing apparatus prescribed in JIS S 6005:2007by a writing load is 0.191 to 0.218.(9) The pencil lead as described in any one of the above items (1) to(8), wherein when a polished cross section of the pencil lead isobserved in an area of 5 μm×5 μm by means of FE-SEM (acceleratingvoltage: 5 kV), 1 to 300 particles of the nanoparticles are observed.(10) A production process for the pencil lead as described in any one ofthe above items (1), (2) and (5) to (9), wherein a lead body of a pencillead containing flake graphite which has an ab face having a flatness ofat least 2 μm or less and in which an aspect ratio of an a axis or a baxis in the ab face to a c axis is 5 or more is formed; thennanoparticles are dispersed in a liquid having a refractive index of 1.3to 1.5 and a viscosity of 7 to 200 mm²/s at 25° C., and the lead body isthen impregnated with the liquid.(11) A production process for the pencil lead as described in any one ofthe above items (3) and (5) to (9), wherein flake graphite which has anab face having a flatness of at least 2 μm or less and in which anaspect ratio of an a axis or a b axis in the ab face to a c axis is 5 ormore is brought into contact with nanoparticles having a my value of0.05 to 2 based on 100 of a volume average diameter (mv value) of thegraphite and having a sphericity of 0.1 to 20 nm; then the nanoparticlesare fixed thereon to prepare a composite; a lead body is formed by usingthe composite graphite and impregnated with a liquid having a refractiveindex of 1.3 to 1.5 and a viscosity of 7 to 200 mm²/s at 25° C.(12) A production process for the pencil lead as described in any one ofthe above items (4) to (9), wherein flake graphite which has an ab facehaving a flatness of at least 2 μm or less and in which an aspect ratioof an a axis or a b axis in the ab face to a c axis is 5 or more isbrought into contact with nanoparticles having a my value of 0.05 to 2based on 100 of a volume average diameter (mv value) of the graphite andhaving a specific surface area of 50 to 800 m²/g; then the nanoparticlesare fixed thereon to prepare a composite; a lead body is formed by usingthe composite graphite and impregnated with a liquid having a refractiveindex of 1.3 to 1.5 and a viscosity of 7 to 200 mm²/s at 25° C.(13) The production process for the pencil lead as described in theabove item (11) or (12), wherein the graphite is brought into contactwith the nanoparticles by electrostatic force.(14) A production process for a pencil lead, wherein flake graphitewhich has an ab face having a flatness of at least 2 μm or less and inwhich an aspect ratio of an a axis or a b axis in the ab face to a caxis is 5 or more and nanoparticles having a my value of 0.05 to 2 basedon 100 of a volume average diameter (mv value) of the graphite andhaving a sphericity of 0.1 to 20 nm are dispersed in a plasticizer or asolvent and kneaded, and then a lead body is formed from the kneadedmatter.(15) A production process for a pencil lead, wherein flake graphitewhich has an ab face having a flatness of at least 2 μm or less and inwhich an aspect ratio of an a axis or a b axis in the ab face to a caxis is 5 or more and nanoparticles having a my value of 0.05 to 2 basedon 100 of a volume average diameter (mv value) of the graphite andhaving specific surface area of 50 to 800 m²/g are dispersed in aplasticizer or a solvent and kneaded, and then a lead body is formedfrom the kneaded matter.(16) The production process for a pencil lead as described in the aboveitem (14) or (15), wherein the nanoparticles are carbon nanoparticles.(17) The production process for a pencil lead as described in the aboveitem (16), wherein the nanoparticles are diamond.(18) The production process for a pencil lead as described in any one ofthe above items (14) to (17), wherein the nanoparticles have a volumeaverage diameter (mv value) of 4 to 100 nm.(19) The production process for a pencil lead as described in any one ofthe above items (14) to (18), wherein a total frictional coefficientobtained by dividing an average value (n=10) of total frictional forcesin lines drawn by a drawing method using a drawing apparatus prescribedin JIS S 6005:2007 by a writing load is 0.191 to 0.218.(20) The production process for a pencil lead as described in any one ofthe above items (14) to (19), wherein a clearance formed between theflake graphites by the nanoparticles is impregnated with a liquid havinga refractive index of 1.3 to 1.5 and a viscosity of 7 to 200 mm²/s at25° C. after forming the lead body.(21) The production process for a pencil lead as described in any one ofthe above items (14) to (20), wherein when a polished cross section ofthe pencil lead is observed in an area of 5 μm×5 μm by means of FE-SEM(accelerating voltage: 5 kV), 1 to 300 nanoparticles are observed.

In the drawing apparatus prescribed in JIS S 6005:2007 used in thepresent invention, a lead body is slanted at an angle of 75 degrees todraw lines while allowing the lead body to rotate on its axis, and adrawing mode is close to a mode in writing and drawing by the foregoingmechanical pencil of a type in which a lead is rotated whenever writtento always carry out writing by a fresh part. In the present invention, avalue (n=10) obtained by dividing an average value of total frictionalforces in lines drawn by a drawing method using a drawing apparatusprescribed in JIS S 6005:2007 by a writing load is set to a dynamicfrictional coefficient, and a value obtained by dividing a frictionalforce in the beginning of writing by a writing load is set to a staticfrictional coefficient to add to evaluation items.

Effects of the Invention

According to the present invention, provided are a pencil lead usingnanoparticles which has better and smoother writing feeling and whichprovides drawn lines with high intensity and a brilliant black color anda production process for the same, wherein the pencil lead is used for amechanical pencil of a type in which a lead is rotated whenever writtento always carry out writing by a fresh part in addition to pencil leadsused for conventional mechanical pencils, wood case pencils and thelike.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an explanatory drawing based on an electron microphotograph(SEM) for measuring a flatness and the like of natural flake graphite.

MODES FOR CARRYING OUT THE INVENTION

The embodiments of the present invention, to be specific, a firstembodiment to a sixth embodiment shall be explained below in detail byevery embodiment.

First Embodiment

A pencil lead in the first embodiment of the present invention is apencil lead containing flake graphite which has an ab face having aflatness of at least 2 μm or less and in which an aspect ratio of an aaxis or a b axis in the ab face to a c axis is 5 or more, whereinnanoparticles having a my value of 0.05 to 2 based on 100 of a volumeaverage diameter (mv value) of the graphite and having a sphericity of0.1 to 20 nm are brought into contact with the ab face of the graphite.

Further, a production process for the pencil lead in the firstembodiment of the present invention is characterized by forming a leadbody of a pencil lead containing flake graphite which has an ab facehaving a flatness of at least 2 μm or less and in which an aspect ratioof an a axis or a b axis in the ab face to a c axis is 5 or more, thendispersing nanoparticles in a liquid having a refractive index of 1.3 to1.5 and a viscosity of 7 to 200 mm²/s at 25° C. and then impregnatingthe pencil lead body with the liquid.

“The present invention” includes the respective embodiments of the firstembodiment to the sixth embodiment, and in a case of the sameconstitution as that of the first embodiment, the explanations thereofshall be omitted by showing the fact to the effect in explanationssubsequent to the first embodiment.

The flake graphite used in the first embodiment of the present inventionhas to have an ab face having a flatness of at least 2 μm or less, andan aspect ratio of the a axis or the b axis in the ab face to the c axishas to be 5 or more. From the viewpoints of the writing feeling and thewriting resistance, preferred is the flake graphite which has an ab facehaving a flatness of at least 0.05 to 2 μm and in which an aspect ratioof an a axis or a b axis in the ab face to a c axis is 5 to 100.

The flake graphite which has a flatness exceeding 2 μm and in which anaspect ratio of an a axis or a b axis to a c axis is less than 5provides a condition which is disadvantageous to lubrication and resultsin increasing friction, and that is not preferred.

In the first embodiment of the present invention, the flake graphitewhich can be used shall not specifically be restricted as long as it isflake graphite which has an ab face having a flatness of at least 2 μmor less and in which an aspect ratio of an a axis or a b axis in the abface to a c axis is 5 or more which are the characteristics describedabove, and it can be selected from, for example, natural graphite,artificial graphite, kish graphite, expandable graphite, expandedgraphite and the like which have the characteristics described above.They may be used alone or in combination of two or more kinds thereof.

The flake graphite in the first embodiment has preferably a volumeaverage diameter (mv value) of 4 to 10 μm from the viewpoints of thestrength and the writing feeling.

The volume average diameter (mv value) in the present invention(including the first embodiment to the sixth embodiment, examplesdescribed later and others) means an average diameter weighted by thevolume on measuring results by a laser diffraction•dispersion method. Ina case of, for example, flake graphite, it can be dry-measured by meansof Microtrac (3100II, manufactured by Nikkiso Co., Ltd.), and in a caseof nanoparticles described later, it can be measured by means ofNanotrac (UPA-EX150 (internal probe type), manufactured by Nikkiso Co.,Ltd.).

In the first embodiment of the present invention, the flake graphitedescribed above is used to form a lead body for forming a pencil lead.The lead body for forming a pencil lead can be formed by subjecting ablend composition for pencil lead containing the flake graphitedescribed above to burning treatment or non-burning treatment.

In the first embodiment, the lead body for forming a pencil lead isprepared by using the blend composition for a pencil lead containing theflake graphite having the characteristics described above, and therespective components such as an extender, a lubricant, a bindercomponent such as a thermoplastic synthetic resin and the like, anorganic solvent and the like can suitably be selected and used forcomponents other than the above flake graphite according to the kinds ofthe pencil lead.

For example, in a case where the pencil lead is a burned pencil lead fora mechanical pencil, at least carbon black and amorphous carbon can beadded in addition to the flake graphite, and in a case of a non-burnedpencil lead, at least oil & fat and waxes can be added. Further, in acase of a burned pencil lead, at least an extender and a ceramic bindingmaterial carbon can be added.

The carbon black which can be used includes, for example, oil furnaceblack, gas furnace black, channel black, thermal black, acetylene black,lamp black, graphitized carbon black which is obtained by graphitizingthese carbon black and the like.

The extender shall not specifically be restricted as long as it is usedfor conventional pencil leads, and any ones can be used. For example,white extenders and color extenders such as boron nitride, kaolin(kaolinite, halloysite), montmorillonite, talc, mica, calcium carbonateand the like can be used as well, and naturally mixtures of plural kindsthereof can be used as well. Particularly preferably, it includes boronnitride, kaolin and talc in terms of physical properties and a formthereof.

The ceramic binding material includes crystalline or amorphous SiO₂,Si₃N₄, Al₂O₃, ZrO₂, MgO, BN, B₂O₃, AlN and the like. They may be usedalone or in combination of two or more kinds thereof.

The thermoplastic synthetic resin includes, for example, polyvinylalcohol, polyvinyl chloride, chlorinated polyvinyl chloride, polyamide,polyethylene, polypropylene, polyetheretherketone and the like.

The organic solvent is preferably a solvent which can dissolve thethermoplastic synthetic resins described above, and to be specific,capable of being used are dioctyl phthalate, dibutyl phthalate,tricresyl phosphate, dioctyl adipate, diallyl isophthalate, propylenecarbonate, alcohols, ketones, esters and the like.

Also, in the burned pencil lead for a mechanical pencil, α-olefinoligomers, fatty acid esters, spindle oils, waxes, boron nitride, talc,silicone oils, silica fine particles, metal soaps and the like can beused as the other components. In the non-burned pencil lead or theburned pencil lead, silicone oils, lard, acryl resins, epoxy resins,celluloid, other thermoplastic resins and the like can be used as theother components.

In the first embodiment of the present invention, the foregoing blendcomposition for a pencil lead comprising the respective components (anextender, a thermoplastic resin, an organic solvent and the like) used,for example, for the burned pencil lead for a mechanical pencil, thenon-burned pencil lead and the burned pencil lead are kneaded, molded,dried and subjected to burning treatment or non-burning treatment(drying at a low temperature of 50 to 120° C.) under non-oxidativeatmosphere, whereby a lead body for a pencil lead can be formed.

A content of the flake graphite having the characteristics describedabove which is used for the above lead body for a pencil lead ispreferably 20 to 80% by mass (hereinafter referred to merely as “%”),more preferably 30 to 70% based on the total amount of the blendcomposition for a pencil lead, and an optimum value thereof is variedaccording to the hardness.

If a content of the flake graphite is less than 20% or exceeds 80%, thatresults in breakage of balance between the hardness, the writing feelingand the strength, and therefore that is not preferred.

In the first embodiment of the present invention, for example, in a caseof producing the burned pencil lead for a mechanical pencil, (a) 20 to80% of the flake graphite having the characteristics described above,(b) 30 to 60% of the thermoplastic synthetic resin and (c) 0 to 30% ofthe organic solvent which can dissolve the thermoplastic synthetic resinbased on the total amount of the blend composition for a pencil leadpreferably in terms of the strength, the intensity and the writingfeeling are dispersed and mixed by means of a Henschel mixer and kneadedby means of a pressure kneader or a double roll mill, and the kneadedmixture is molded by means of an extrusion molding machine; and themolded matter is then dried at 110 to 250° C. in an electric furnace andsubsequently burned at 800 to 1400° C. for 20 to 40 hours undernon-oxidative atmosphere (under nitrogen gas atmosphere or inert gasatmosphere), whereby a lead body for forming a pencil lead can beformed.

The pencil lead in the first embodiment of the present invention isobtained by dispersing nanoparticles in a liquid having a refractiveindex of 1.3 to 1.5 and a viscosity of 7 to 200 mm²/s at 25° C. and thenimpregnating the pencil lead body formed above with the liquid.

The liquid used in the first embodiment of the present invention is usedto build a structure in which the pores of the pencil lead body areimpregnated with the nanoparticles to bring the nanoparticles intocontact with an ab face of the flake graphite constituting the pencillead body and to have the nanoparticles work as a lubricant as well asthe purpose of enhancing the density of them, and it includes liquidshaving a refractive index of 1.3 to 1.5 and a viscosity of 7 to 200mm²/s at 25° C. in terms of easiness of impregnation thereof into thepores and a reflectance of light.

The liquid which can be used shall not specifically be restricted aslong as it is a liquid having the characteristics described above, andit includes a single component or a mixture of two or more kinds ofdimethyl silicone, dimethyl silicone oil, carboxymethyl cellulose (CMC)liquids, trimethylpentaphenyltrisiloxane, liquid paraffins, fatty acidesters and the like each having the characteristics described above. Tobe specific, it includes Hicall M series manufactured by Kaneda Co.,Ltd., KF-96 series manufactured by Shin-Etsu Chemical Co., Ltd. andothers which are commercially available.

The refractive index in the present invention (including examples andthe like described later) means an absolute refractive index, and thedynamic viscosity means a value of a unit (mm²/s) based on viscositymeasuring methods prescribed in JIS K 2283 and JIS Z 8803 and can bemeasured directly by, for example, “Cannon Fenske” and “Ubbelohde”.

When a refractive index of the above liquids is less than 1.3 or exceeds1.5, contribution to reduction in the reflectance is small. Further, ifa viscosity thereof is less than 7 mm²/s, the liquid can not be retainedin the lead body and flows out with the passage of time. On the otherhand, if the viscosity exceeds 200 mm²/s, the liquid is not penetratedevenly into the pores, and that is not preferred.

In the first embodiment of the present invention, the nanoparticles usedshall not specifically be restricted, and any ones can be used as longas they are usually classified into nanoparticles. Capable of being usedare, for example, diamond nanoparticles, carbon nanoparticles such ascomposite particles of carbon nanotubes and composite particles offullerene, oxide ceramics of metals such as silicon, titanium,zirconium, aluminum, cerium and ceramic nanoparticles of nitrideceramics, phosphate ceramics, carbide ceramics, silicate ceramics,boride ceramics and the like.

When producing the pencil lead, carbon nanoparticles are preferred fromthe viewpoint of inhibiting a change in the hue, and diamondnanoparticles are particularly preferred from the viewpoint of obtainingthe economical efficiency and the smooth writing property.

The diamond nanoparticles which can be used include, for example,diamond nanoparticles prepared by an explosion method, a hydrostaticmethod, an impact compression method, an EACVD method, a gas phasesynthetic method and a liquid phase growing method, and a form thereofincludes, for example, polycrystalline diamond particles, single crystaldiamond particles, cluster diamond and the like.

To be specific, capable of being used are a trade name “NanoAmando B”manufactured by NanoCarbon Research Institute Co., Ltd., MD seriesmanufactured by Tomei Diamond Co., Ltd., SCM Nanodiamond and SCM FineDiamond manufactured by Sumiseki Materials Co., Ltd., CD (ClusterDiamond), CDS (Cluster Diamond Slurry), GCD (Graphite Cluster Diamond)and GCDS (Graphite Cluster Diamond Slurry) each manufactured byNanotec-Systems Inc. and artificial diamonds manufactured by JERO Co.,Ltd. and the like.

A sphericity of the nanoparticles used falls in a range of 1 to 20 nm,preferably 0.1 to 10 nm and more preferably 0.1 to 5 nm. In the presentinvention (including the first embodiment to the sixth embodiment,examples described later and others), “the sphericity” means the sameone as prescribed in JIS B 1501 as a method for measuring a steel ballfor a ball bearing. According to the method, the sphericity isdetermined by measuring contours of a ball surface of two or threeequatorial planes of one steel ball to be measured which are crossedwith each other at 90° by means of a roundness measuring machine anddetermining a maximum value of distances from the respective minimumcircumscribed circles to the surface of the steel ball in a radialdirection. Since the nanoparticles used in the first embodiment of thepresent invention are too fine, a sphericity thereof is not measured bythe above method, and therefore it is to be measured based on JIS. Theroundness is measured by image treatment to set it to a value of thesphericity, wherein the roundness is measured as a maximum value ofdistances from the minimum circumscribed circles to the surface of thesteel ball in a radial direction only on one equatorial plane of tenparticles observed on a SEM or TEM image.

The nanoparticles having a sphericity of less than 1 nm are notpreferred in terms of an availability of the raw material, a cost, ahandling property and the like. On the other hand, if the nanoparticleshaving a sphericity of exceeding 20 nm are used, the probability that ashape of the nanoparticles themselves is unsuitable as a solid lubricantis increased to bring about the steric hindrance, and that results inincreasing the friction and is not preferred.

In the first embodiment of the present invention, a volume averagediameter (my value) of the nanoparticles used has to have a my value of0.01 to 2 based on 100 of a volume average diameter (mv value) of thegraphite having the characteristics described above in the nanoparticlessuch as the nanoparticles comprising the ceramic materials describedabove and the carbon nanoparticles including the diamond nanoparticlesin terms of combining pores (closed cells) with pores (closed cells) inthe pencil lead in the production and further forming pores (open cells)of an open type, and it has preferably a my value of 0.1 to 1.

A volume average diameter (mv value) of the nanoparticles used ispreferably 4 to 100 nm, more preferably 5 to 40 nm and particularlypreferably 5 to 30 nm.

If a volume average diameter (my value) of the nanoparticles such as thenanomaterials comprising the ceramic materials and the carbonnanoparticles including the diamond nanoparticles is less than 0.05based on 100 of a volume average diameter (mv value) of the graphitehaving the characteristics described above or a volume average diameter(mv value) of the nanoparticles is less than 4 nm, the particles aredifficult to be monodispersed and liable to be coagulated, and they areincreased in reactivity and become instable to result in exerting aninverse action on sliding of the graphite. On the other hand, if itexceeds 2 based on 100 of a volume average diameter (mv value) of thegraphite having the characteristics described above or a volume averagediameter (mv value) of the nanoparticles exceeds 100 nm, the pencil leadis broken in a structure and reduced in strength, and that is notpreferred.

A trace amount of impurities is contained in the diamond nanoparticlesdescribed above, and almost all of them are sp³ surface functionalgroups originating in the diamond structure and are components removedin dispersing of nanoparticles in oil. Impurities other than themaccount for about 0.2% and therefore do not exert an adverse influenceon the effects of the present invention. Also, diamond having “a diamondpurity of 99% or more” is a solid lubricant having a low frictionalcoefficient, and usually if impurities which are not a solid lubricantexceed 1% in a solid lubricant, a lubricating characteristic thereofstarts being deteriorated.

A content of the nanoparticles having the above characteristics in theliquid is controlled so that a content of the nanoparticles in thepencil lead obtained by impregnation treatment is preferably 0.001 to5%, more preferably 0.002 to 1% and particularly preferably 0.01 to0.5%.

In order to control a content of the nanoparticles in the pencil leadobtained to the ranges described above, the nanoparticles are contained,though varied depending on a size, a pore diameter and a pore volume ofthe lead body, in a proportion of preferably 0.01 to 10%, morepreferably 0.02 to 2% and particularly preferably 0.05 to 0.5% base onthe total amount of the liquid used for impregnation treatment.

If a content of the nanoparticles in the above pencil lead is less than0.001%, an effective pore volume thereof is scarcely varied, and adifference from a pencil lead to which the nanoparticles are not addedis not observed. On the other hand, in order to prepare a lead in whicha content of the nanoparticles exceeds 5%, an effective pore volumethereof has to be increased, but it allows the pencil lead to be notablyreduced in strength. Further, a concentration of the nanoparticles inthe dispersion for impregnation has to be enhanced, but it brings aboutscattering in a distribution amount of the nanoparticles in the leadbody, and that is not preferred.

In the first embodiment of the present invention, the lead body isdipped as it is in a dispersion obtained by dispersing the nanoparticleshaving the characteristics described above in the liquid having thecharacteristics described above or subjected to dipping treatment andthe like under applying pressure (for example, 0.5 to 5 MPa) and/orunder heating (for example, liquid temperature of 60 to 200° C.),whereby obtained is the targeted pencil lead, that is, the pencil leadwhich contains the flake graphite having the characteristics describedabove and in which the nanoparticles having a my value of 0.05 to 2based on 100 of a volume average diameter (mv value) of the graphite andhaving a sphericity of 0.1 to 20 nm are brought into contact with the abface of the above graphite.

The pencil lead obtained in the first embodiment contains thenanoparticles in the range described above, and it is a pencil leadprovided with a suited abrasion characteristic by producing it by theproduction process described above. More preferred is the pencil lead inwhich a total frictional coefficient (dynamic frictional coefficient)obtained by dividing an average value (n=10) of total frictional forcesin lines drawn by a drawing method using a drawing apparatus prescribedin JIS S 6005:2007 by a writing load is 0.191 to 0.218, and this makesit possible to obtain a lead by which smoother writing is felt as wellin a mechanical pencil lead of a type in which a lead is rotated.

Also, when a polished cross section of the pencil lead is observed in anarea of 5 μm×5 μm by means of FE-SEM (S-4700 model, acceleratingvoltage: 5 kV, current value: 10 μA, manufactured by HitachiHigh-Technologies Corporation), 1 to 300 nanoparticles are preferablyobserved. In addition of the nanoparticles in “the more preferred range”described above, 2 to 100 particles are observed, and in additionthereof in “the particularly preferred range”, 5 to 50 particles areobserved.

The above total frictional coefficient and the number of thenanoparticles can be controlled by suitably combining the physicalproperties of the flake graphite used such as a flatness, an aspectratio and a content thereof and a sphericity, a volume average diameter(mv value) and a content (impregnation amount) of the nanoparticles andthe kind of the oil.

In the first embodiment of the present invention thus constituted, ablend composition for a pencil lead containing at least the flakegraphite which has an ab face having a flatness of at least 2 μm or lessand in which an aspect ratio of an a axis or a b axis in the ab face toa c axis is 5 or more is subjected to burning treatment, and then it isimpregnated with the liquid containing the nanoparticles having asphericity of 0.1 to 20 nm and a volume average diameter (mv value)falling in a specific range based on the graphite having thecharacteristics described above, whereby prepared is a state in whichthe nanoparticles are penetrated into the pores of a porous bodyconstituted from the flake graphite having the characteristics describedabove (a state in which the nanoparticles are brought into contact withthe ab face of the flake graphite). Thus, the properties of the pencillead having a porous structure are different from those of conventionalones. To be specific, the nanoparticles contained in the liquid havingthe characteristics described above exert an effect of a suspension or abearing, and therefore the lead is improved in lubrication to a largerextent than in a case where the nanoparticles are not added. This allowsthe pencil lead to be improved in lubrication to a large extent. Sincepenetration of the nanoparticles into the lead body allows the smoothdrawn lines to bring about diffused reflection, so-called “shine”disappears, and it results in making the color dense. In additionthereto, friction between paper and the flake graphite particles andbetween the flake graphite particles themselves are reduced by theaction of the flake graphite itself having the characteristics describedabove, and the erasability is enhanced as well. Further, since thenanoparticles can evenly be dispersed without disturbing orientation ofthe flake graphite having the characteristics described above, an effectthereof as an extender is exerted, and the compression strength isenhanced as well. Also, since an abrasion amount of the lead is small,an amount of the graphite contained in the drawn lines is small as well,and the fingers are less liable to be stained.

Further, in the first embodiment of the present invention, prepared is apencil lead having, in addition to the action and effect describedabove, drawn line intensity, writing feeling and a low static•dynamicfrictional coefficient which exceed those of the pencil leads disclosedin patent document 1 described above. In particular, even in a case of apencil lead used for a mechanical pencil of a type in which a lead isrotated whenever written to always carry out writing by a fresh part,obtained are a pencil lead which has better and smoother writing feelingand which provides drawn lines with higher intensity and a brilliantblack color and a production process for the same (this matter shall beexplained in further details in Examples 1 to 6, Reference Example 1 andComparative Examples 1 to 7 each described later).

The pencil lead prepared in the first embodiment of the presentinvention shall not be restricted to the first embodiment describedabove and can be varied in various manners and in the scope of thetechnical concept of the present invention. For example, it may be apencil lead prepared such that the nanoparticles are adhered to an abface of the graphite particles before forming the lead body obtained inthe first embodiment described above, that is, the lead body prepared bysubjecting a blend composition for a pencil lead containing at least theflake graphite having the characteristics described above to burningtreatment or non-burning treatment, and then a lead body is formed andsubjected to burning and impregnating treatment with a liquid containingthe nanoparticles having the characteristics described above. In thiscase, the nanoparticles contained in the lead body and the nanoparticlescontained in the liquid are completely independent from each other, andtherefore the same or different nanoparticles may be used in differentamounts. In this case, a preferred content of the nanoparticles ismaximum 10% in the pencil lead.

Second Embodiment

A pencil lead which is the second embodiment of the present invention isa pencil lead containing flake graphite which has an ab face having aflatness of at least 2 μm or less and in which an aspect ratio of an aaxis or a b axis in the ab face to a c axis is 5 or more, whereinnanoparticles having a my value of 0.05 to 2 based on 100 of a volumeaverage diameter (mv value) of the graphite and having a specificsurface area of 50 to 800 m²/g are brought into contact with the ab faceof the graphite.

Further, a production process for the pencil lead in the secondembodiment of the present invention is characterized by forming a leadbody of a pencil lead containing flake graphite which has an ab facehaving a flatness of at least 2 μm or less and in which an aspect ratioof an a axis or a b axis in the ab face to a c axis is 5 or more, thendispersing nanoparticles in a liquid having a refractive index of 1.3 to1.5 and a viscosity of 7 to 200 mm²/s at 25° C. and then impregnatingthe above pencil lead body with the liquid.

The second embodiment of the present invention is different only interms of using the nanoparticles having a my value of 0.05 to 2 based on100 of a volume average diameter (mv value) of the flake graphite andhaving a specific surface area of 50 to 800 m²/g in place of thenanoparticles used in the first embodiment described above which have amy value of 0.05 to 2 based on 100 of a volume average diameter (mvvalue) of the flake graphite and which have a sphericity of 0.1 to 20nm, and use of the flake graphite other than the nanoparticles(described later) and formation of the lead body for forming a pencillead by using the flake graphite are the same as in the first embodimentdescribed above. Points different from the first embodiment describedabove shall be described below in detail, and explanations of the sameconstitution, production process and the like shall be omitted.

The pencil lead in the second embodiment of the present invention isobtained by forming a lead body in the same manner as in the firstembodiment described above, dispersing the nanoparticles having a myvalue of 0.05 to 2 based on 100 of a volume average diameter (mv value)of the graphite and having a specific surface area of 50 to 800 m²/g inthe same liquid having a refractive index of 1.3 to 1.5 and a viscosityof 7 to 200 mm²/s at 25° C. as in the first embodiment and thenimpregnating the lead body with the liquid.

In the second embodiment, the nanoparticles used shall not specificallybe restricted as long as they are usually classified into nanoparticlesand have the characteristics described above, and any ones can be used.Capable of being used are, for example, diamond nanoparticles, carbonnanoparticles such as composite particles of carbon nanotubes andcomposite particles of fullerene, oxide ceramics of metals such assilicon, titanium, zirconium, aluminum, cerium and ceramic nanoparticlesof nitride ceramics, phosphate ceramics, carbide ceramics, silicateceramics, boride ceramics and the like.

When producing the pencil lead, carbon nanoparticles are preferred fromthe viewpoint of inhibiting a change in the hue, and diamondnanoparticles are particularly preferred from the viewpoint of obtainingthe economical efficiency and the smooth writing property.

The diamond nanoparticles which can be used include, for example,diamond nanoparticles prepared by an explosion method, a hydrostaticmethod, an impact compression method, an EACVD method, a gas phasesynthetic method and a liquid phase growing method, and a form thereofincludes, for example, polycrystalline diamond particles, single crystaldiamond particles, cluster diamond and the like.

To be specific, capable of being used are a trade name “NanoAmando B”manufactured by NanoCarbon Research Institute Co., Ltd., MD seriesmanufactured by Tomei Diamond Co., Ltd., SCM Nanodiamond and SCM FineDiamond manufactured by Sumiseki Materials Co., Ltd., CD (ClusterDiamond), CDS (Cluster Diamond Slurry), GCD (Graphite Cluster Diamond)and GCDS (Graphite Cluster Diamond Slurry) each manufactured byNanotec-Systems Inc., artificial diamonds manufactured by JERO Co., Ltd.and the like.

A specific surface area of the nanoparticles used in the secondembodiment falls in a range of 50 to 800 m²/g, preferably 100 to 800m²/g and more preferably 300 to 700 m²/g. In the present invention(including examples described later and others), “the specific surfacearea” of the nanoparticles is shown by a BET specific surface areadetermined by a BET dynamic method. The BET specific surface area can bemeasured by means of, for example, a full automatic BET specific surfacearea measuring apparatus (HM model-1208) manufactured by Mountech Co.,Ltd.

In the nanoparticles having a specific surface area of less than 50m²/g, nano-lubrication in a grain boundary of the graphite which isproposed in the present invention is not obtained, and it is notpreferred in terms of availability of the raw materials, the cost, thehandling property and the like. On the other hand, if the nanoparticleshaving a specific surface area of exceeding 800 m²/g are used, theprobability that a form of the nanoparticles themselves are unsuitableas a solid lubricant is increased, and sliding lubrication which isproposed in the present invention is not obtained, so that that is notpreferred.

In the second embodiment of the present invention, volume averagediameter (my value) of the nanoparticles used has to have, as is thecase with the first embodiment described above, a my value of 0.01 to 2based on 100 of a volume average diameter (my value) of the graphitehaving the characteristics described above in the nanoparticles such asthe nanoparticles comprising the ceramic materials described above andthe carbon nanoparticles including the diamond nanoparticles in terms ofcombining pores (closed cells) with pores (closed cells) in the pencillead in the production and further forming pores (open cells) of an opentype, and it is preferably a my value of 0.1 to 1.

A volume average diameter (my value) of the nanoparticles used is, as isthe case with the first embodiment described above, preferably 4 to 100nm, more preferably 5 to 40 nm and particularly preferably 5 to 30 nm.

A trace amount of impurities is contained in the diamond nanoparticlesdescribed above, and almost all of them are sp³ surface functionalgroups originating in the diamond structure and are components removedin dispersing the nanoparticles in oil. Impurities other than themaccount for about 0.2% and therefore do not exert an adverse influenceon the effects of the present invention. Also, diamond having “a diamondpurity of 99% or more” is a solid lubricant having a low frictionalcoefficient, and usually if impurities which are not the solid lubricantexceed 1% in the solid lubricant, a lubricating characteristic thereofstarts being deteriorated.

A content of the nanoparticles having the above characteristics in theliquid is controlled, as is the case with the first embodiment, so thata content of the nanoparticles in the pencil lead obtained byimpregnation treatment is preferably 0.001 to 5%, more preferably 0.002to 1% and particularly preferably 0.01 to 0.5%.

In order to control a content of the nanoparticles in the pencil leadobtained to the ranges described above, the nanoparticles are contained,though varied depending on a size, a pore diameter and a pore volume ofthe lead body, in a proportion of preferably 0.01 to 10%, morepreferably 0.02 to 2% and particularly preferably 0.05 to 0.5% based onthe total amount of the liquid used for impregnation treatment.

If a content of the nanoparticles in the above pencil lead obtained isless than 0.001%, an effective pore volume thereof is scarcely varied,and a difference from a pencil lead to which the nanoparticles are notadded is not observed. On the other hand, in order to prepare a lead inwhich a content of the nanoparticles exceeds 5%, an effective porevolume thereof has to be increased, but it allows the pencil lead to benotably reduced in strength. Further, a concentration of thenanoparticles in the dispersion for impregnation has to be enhanced, butit brings about scattering in a distribution amount of the nanoparticlesin the lead body, and that is not preferred.

In the second embodiment of the present invention, the pencil lead bodyis dipped as it is in a dispersion obtained by dispersing thenanoparticles having the characteristics described above in the liquidhaving the characteristics described above or subjected to dippingtreatment and the like under applying pressure (for example, 0.5 to 5MPa) and/or under heating (for example, liquid temperature: 60 to 200°C.), whereby obtained is the targeted pencil lead, that is, the pencillead which contains the flake graphite having the characteristicsdescribed above and in which the nanoparticles having a my value of 0.05to 2 based on 100 of a volume average diameter (mv value) of thegraphite and having a specific surface area of 50 to 800 m²/g arebrought into contact with the ab face of the graphite.

The pencil lead obtained contains the nanoparticles in the rangedescribed above, and it is turned into a pencil lead provided with asuited abrasion characteristic and the like by producing it according tothe production process described above. More preferred is the pencillead in which a total frictional coefficient (dynamic frictionalcoefficient) obtained by dividing an average value (n=10) of totalfrictional forces in lines drawn by a drawing method using the drawingapparatus prescribed in JIS S 6005:2007 by a writing load is 0.191 to0.218, and this makes it possible to obtain a lead by which smootherwriting is felt as well in a mechanical pencil lead of a type in which alead is rotated.

Also, when a polished cross section of the pencil lead is observed in anarea of 5 μm×5 μm by means of FE-SEM (S-4700 model, acceleratingvoltage: 5 kV, current value: 10 μA, manufactured by HitachiHigh-Technologies Corporation), 1 to 300 nanoparticles are preferablyobserved. In addition of the nanoparticles in “the more preferred range”described above, 2 to 100 particles are observed, and in additionthereof in “the particularly preferred range”, 5 to 50 particles areobserved.

The above total frictional coefficient and the number of thenanoparticles can be controlled by suitably combining the physicalproperties of the flake graphite used such as a flatness, an aspectratio and a content thereof and a specific surface area, a volumeaverage diameter (mv value) and a content (impregnation amount) of thenanoparticles and the kind of the oil.

In the second embodiment of the present invention thus constituted, ablend composition for a pencil lead containing at least the flakegraphite which has an ab face having a flatness of at least 2 μm or lessand in which an aspect ratio of an a axis or a b axis in the ab face toa c axis is 5 or more is subjected to burning treatment, and then it isimpregnated with the liquid containing the nanoparticles having aspecific surface area of 50 to 800 m²/g and a volume average diameter(mv value) falling in a specific range based on the graphite having thecharacteristics described above, whereby prepared is a state in whichthe nanoparticles are penetrated into the pores of a porous bodyconstituted from the flake graphite having the characteristics describedabove (a state in which the nanoparticles are brought into contact withthe ab face of the flake graphite). Thus, the properties of the pencillead having a porous structure are different from those of conventionalone. To be specific, the nanoparticles contained in the liquid havingthe characteristics described above exert an effect of a suspension or abearing, and therefore the lead is improved in lubrication to a largerextent than in a case where the nanoparticles are not added. This allowsthe pencil lead to be improved in lubrication to a large extent. Sincepenetration of the nanoparticles into the lead body allows the smoothdrawn lines to bring about diffused reflection, so-called “shine”disappears, and it results in obtaining the dense color. In additionthereto, friction between paper and the flake graphite particles andbetween the flake graphite particles themselves are reduced by theaction of the flake graphite itself having the characteristics describedabove, and the erasability is enhanced. Further, since the nanoparticlescan evenly be dispersed without disturbing orientation of the flakegraphite having the characteristics described above, an effect thereofas an extender is exerted, and the compression strength is enhanced aswell. Also, since an abrasion amount of the lead is small, an amount ofthe graphite contained in the drawn lines is small as well, and thefingers are less liable to be stained.

Further, in the present invention, prepared is a pencil lead having, inaddition to the action and effect described above, drawn line intensity,writing feeling and a low static•dynamic frictional coefficient whichexceed those of the pencil leads disclosed in patent document 1. Inparticular, even in a case of a pencil lead used for a mechanical pencilof a type in which a lead is rotated whenever written to always carryout writing by a fresh part, obtained are a pencil lead which has betterand smoother writing feeling and which provides drawn lines with higherintensity and a brilliant black color and a production process for thesame (this matter shall be explained in further details in Examples 7 to12, Reference Example 2 and Comparative Examples 8 to 15 each describedlater).

The pencil lead prepared in the second embodiment of the presentinvention shall not be restricted to the second embodiment describedabove and can be varied in various manners and in the scope of thetechnical concept of the present invention. For example, it may be apencil lead prepared such that the nanoparticles are adhered to an abface of the graphite particles before forming the lead body obtained inthe second embodiment described above, that is, the lead body preparedby subjecting a blend composition for a pencil lead containing at leastthe flake graphite having the characteristics described above to burningtreatment or non-burning treatment, and then a lead body is formed andsubjected to burning and impregnating treatment with a liquid containingthe nanoparticles having the characteristics described above. In thiscase, the nanoparticles contained in the lead body and the nanoparticlescontained in the liquid are completely independent from each other, andtherefore the same or different nanoparticles may be used in differentamounts. In this case, a preferred content of the nanoparticles ismaximum 10% in the pencil lead.

Third Embodiment

A pencil lead which is the third embodiment of the present invention isa pencil lead containing flake graphite which has an ab face having aflatness of at least 2 μm or less and in which an aspect ratio of an aaxis or a b axis in the ab face to a c axis is 5 or more, whereinnanoparticles having a my value of 0.05 to 2 based on 100 of a volumeaverage diameter (mv value) of the graphite and having a sphericity of0.1 to 20 nm are adhered to the ab face of the graphite.

Further, a production process for the pencil lead in the thirdembodiment of the present invention is characterized by bringing flakegraphite which has an ab face having a flatness of at least 2 μm or lessand in which an aspect ratio of an a axis or a b axis in the ab face toa c axis is 5 or more into contact with nanoparticles having a my valueof 0.05 to 2 based on 100 of a volume average diameter (my value) of thegraphite and having a sphericity of 0.1 to 20 nm, then fixing thenanoparticles thereon to prepare a composite, forming a lead body byusing the composite graphite and impregnating it with a liquid having arefractive index of 1.3 to 1.5 and a viscosity of 7 to 200 mm²/s at 25°C.

In the third embodiment of the present invention, the flake graphite andthe nanoparticles each having the same physical properties as in thefirst embodiment are used, and it is different only in that thenanoparticles having a my value of 0.05 to 2 based on 100 of a volumeaverage diameter (my value) of the graphite and having a sphericity of0.1 to 20 nm are not brought into contact with but are adhered to the abface of the graphite, and use of the flake graphite and thenanoparticles other than the above constitution are the same as in thefirst embodiment described above. Points different from the firstembodiment shall be described below in detail, and explanations of thesame constitution and the like shall be omitted.

In the third embodiment of the present invention, the flake graphitehaving the same characteristics as in the first embodiment is broughtinto contact with the nanoparticles; then the nanoparticles are fixedthereon to prepare a composite; and a lead body for a pencil lead isthen formed by using the composite graphite.

The lead body for a pencil lead in the third embodiment can be formed bysubjecting a blend composition for a pencil lead containing thecomposite graphite on which the nanoparticles are fixed to burningtreatment or non-burning treatment.

In the third embodiment of the present invention, before forming thelead body for a pencil lead, the above flake graphite is brought intocontact with the nanoparticles having the characteristics describedabove, and then the composite graphite on which the nanoparticles arefixed is prepared. As a result, the nanoparticles are dispersed on awhole part of the lead.

A method for preparing the composite graphite on which the nanoparticlesare fixed includes, for example, a method in which the graphite isbrought into contact with the nanoparticles by virtue of electrostaticforce (electrostatic adhesion) to prepare a nanoparticle-fixed compositegraphite, a method in which the nanoparticles are dispersed and adheredto the graphite by virtue of van der Waal's force to prepare ananoparticle-fixed composite graphite and a method in which thenanoparticles are added while crushing the graphite to result inadhering the nanoparticles to the graphite by virtue of cohesive forceexerted by van der Waal's force and the like.

The electrostatic adhesion described above is a method in which thenanoparticles are bonded with the graphite by accepting and donatingelectrons, and the disperse adhesion carried out by virtue of the vander Waal's force described above is a method in which the nanoparticlesare bonded with the graphite by virtue of intermolecular forceoriginating in a polarizing action in the graphite and thenanoparticles.

To be specific, it can be prepared by putting the flake graphite and thenanoparticles each described above into a Henschel mixer rotated at ahigh speed to prepare a nanoparticle-fixed composite graphite, puttingan aqueous dispersion of the nanoparticles and the flake graphite into aHenschel mixer rotated at a high speed and vaporizing water with heatgenerated by frictional force between the particles to prepare ananoparticle-fixed composite graphite, putting the graphite into a NPA(n-propyl alcohol) dispersion of the nanoparticles to carry out disperseadhesion by dry coating, applying a polycation agent on the graphite tocarry out electrostatic adhesion to prepare a composite of thenanoparticles or mixing the graphite and the nanoparticles by means of avibration ball mill to prepare a composite of the nanoparticles whilecrushing the graphite.

In terms of the adhesive force and the lubrication effect, the graphitehaving the characteristics described above is preferably brought intocontact with the nanoparticles having the characteristics describedabove by virtue of electrostatic force (electrostatic adhesion) toprepare a nanoparticle-fixed composite graphite.

The flake graphite having the characteristics described above which isused for forming the above lead body for a pencil lead is turned to ananoparticle-fixed composite graphite before forming the lead body andused, and a content of the flake graphite is preferably 20 to 80%, morepreferably 30 to 70% based on the total amount of the blend compositionfor a pencil lead. An optimum value thereof is varied according to thehardness.

If a content of the above flake graphite is less than 20% or exceeds80%, that results in breakage of balance between the hardness, thewriting feeling and the strength, and therefore that is not preferred.

Further, the nanoparticles having the characteristics described aboveare used in the form of a nanoparticle-fixed composite graphite, and acontent thereof is controlled so that a content of the nanoparticles inthe pencil lead obtained by impregnation treatment is preferably 0.001to 5%, more preferably 0.01 to 1% and particularly preferably 0.02 to0.5% based on the total amount of the blend composition for a pencillead.

If a content of the nanoparticles is less than 0.001%, an effective porevolume thereof is scarcely varied, and a difference from a pencil leadto which the nanoparticles are not added is not observed. On the otherhand, if a content of the nanoparticles exceeds 5%, an effective porevolume thereof is increased, but the pencil lead is broken in astructure and reduced in strength, and therefore that is not preferred.

In the third embodiment of the present invention, the lead body for apencil lead is prepared by using the blend composition containing for apencil lead the composite graphite on which the nanoparticles having thecharacteristics described above are fixed, and the respective componentssuch as an extender, a lubricant, a binder component such as athermoplastic synthetic resin, an organic solvent and the like cansuitably be selected, and used for components other than the abovecomposite graphite on which the nanoparticles are fixed according to thekinds of the pencil lead as is the case with the first embodiment.

In the third embodiment of the present invention, the foregoing blendcomposition for a pencil lead comprising the respective components (anextender, a thermoplastic resin, an organic solvent and the like) used,for example, for the burned pencil lead for a mechanical pencil, thenon-burned pencil lead and the burned pencil lead are kneaded, molded,dried and subjected to burning treatment or non-burning treatment(drying at a low temperature of 50 to 120° C.) under non-oxidativeatmosphere, whereby a lead body for a pencil lead can be formed.

In the third embodiment of the present invention, for example, in a caseof producing the burned pencil lead for a mechanical pencil, preferablyin terms of the strength, the intensity and the writing feeling, as isthe case with the first embodiment, (a) 20 to 80% of the flake graphitehaving the characteristics described above and (b) 0.01 to 5% of thenanoparticles having the characteristics described above which arebrought into contact with the flake graphite having the characteristicsdescribed above and are then fixed thereon to prepare a composite; (c)30 to 60% of the thermoplastic synthetic resin and (d) 0 to 30% of theorganic solvent which can dissolve the above thermoplastic syntheticresin based on the total amount of the blend composition for a pencillead are dispersed and mixed by means of a Henschel mixer and kneaded bymeans of a pressure kneader or a double roll mill, and the kneadedmixture is molded by means of an extrusion molding machine; and themolded matter is then dried at 110 to 250° C. in an electric furnace andsubsequently burned at 800 to 1400° C. for 20 to 40 hours undernon-oxidative atmosphere (under nitrogen gas atmosphere or inert gasatmosphere), whereby a lead body for forming a pencil lead can beformed.

The pencil lead in the third embodiment of the present invention isobtained by impregnating, as is the case with the first embodiment, thelead body formed above with a liquid having a refractive index of 1.3 to1.5 and a viscosity of 7 to 200 mm²/s at 25° C.

In the third embodiment of the present invention, the pencil lead isdipped as it is in the liquid having the characteristics described aboveor subjected to dipping treatment and the like under applying pressure(for example, 0.5 to 5 MPa) and/or under heating (for example, liquidtemperature: 60 to 200° C.), whereby obtained is the targeted pencillead, that is, the pencil lead which contains the graphite having thecharacteristics described above and in which the nanoparticles having amy value of 0.05 to 2 based on 100 of a volume average diameter (mvvalue) of the graphite and having a sphericity of 0.1 to 20 nm areadhered to the ab face of the graphite.

The pencil lead obtained is turned into a pencil lead provided with asuited abrasion characteristic and the like by producing it according tothe production process described above. More preferred is the pencillead in which a dynamic frictional coefficient obtained by dividing anaverage value (n=10) of total frictional forces in lines drawn by adrawing method using a drawing apparatus prescribed in JIS S 6005:2007by a writing load is 0.191 to 0.218, and this makes it possible toobtain a lead by which smoother writing is felt as well in a mechanicalpencil lead of a type in which a lead is rotated.

Also, when a polished cross section of the pencil lead is observed in anarea of 5 μm×5 μm by means of FE-SEM (S-4700 model, acceleratingvoltage: 5 kV, current value: 10 μA, manufactured by HitachiHigh-Technologies Corporation), 1 to 300 nanoparticles are preferablyobserved. In addition of the nanoparticles in “the more preferred range”described above, 2 to 100 particles are observed, and in additionthereof in “the particularly preferred range”, 5 to 50 particles areobserved.

The above total frictional coefficient and the number of thenanoparticles can be controlled by suitably combining the physicalproperties of the flake graphite used such as a flatness, an aspectratio and a content thereof and a sphericity, a volume average diameter(mv value) and a content (impregnation amount) of the nanoparticles andthe kind of the oil.

In the third embodiment of the present invention thus constituted, theflake graphite which has an ab face having a flatness of at least 2 μmor less and in which an aspect ratio of an a axis or a b axis in the abface to a c axis is 5 or more is brought into contact with thenanoparticles having a my value of 0.05 to 2 based on 100 of a volumeaverage diameter (mv value) of the graphite and having a sphericity of0.1 to 20 nm; then the nanoparticles are fixed thereon to prepare acomposite; a lead body is formed by using a blend composition for apencil lead containing the composite graphite, and the lead body for apencil lead is then impregnated with the liquid having thecharacteristic described above, whereby prepared is a state in which thenanoparticles are penetrated into the pores of a porous body constitutedfrom the flake graphite having the characteristics described above (astate in which the nanoparticles are adhered to the ab face of the flakegraphite). Thus, the properties of the pencil lead having a porousstructure are different from those of conventional ones. To be specific,the nanoparticles having the characteristics described above play a roleof a suspension or a bearing, and therefore the lead is improved inlubrication to a larger extent than in a case where the nanoparticlesare not added. This allows the pencil lead to be improved in lubricationto a large extent. Further, since penetration of the nanoparticles intothe lead body allows the smooth drawn lines to bring about diffusedreflection, so-called “shine” disappears, and it results in making thecolor dense. In addition thereto, friction between paper and the flakegraphite particles and between the flake graphite particles themselvesare reduced by the action of the flake graphite itself having thecharacteristics described above, and the erasability is enhanced.Further, since the nanoparticles can evenly be dispersed withoutdisturbing orientation of the flake graphite having the characteristicsdescribed above, an effect thereof as an extender is exerted, and thecompression strength is enhanced as well. Also, since an abrasion amountof the lead is small, an amount of the graphite contained in the drawnlines is small as well, and the fingers are less liable to be stained.

Further, in the third embodiment of the present invention, prepared is apencil lead having, in addition to the action and effect describedabove, drawn line intensity, writing feeling and a low static•dynamicfrictional coefficient which exceed those of the pencil leads disclosedin patent document 1 described above. In particular, even in a case of apencil lead used for mechanical pencils of a type in which a lead isrotated whenever written to always carry out writing by a fresh part,obtained are a pencil lead which has better and smoother writing feelingand which provides drawn lines with higher intensity and a brilliantblack color and a production process for the same (this matter shall beexplained in further details in Examples 13 to 18 and ComparativeExamples 16 to 22 each described later).

The pencil lead prepared in the third embodiment of the presentinvention shall not be restricted to the third embodiment describedabove and can be varied in various manners and in the scope of thetechnical concept of the present invention. For example, it may be apencil lead prepared by filling a liquid containing the nanoparticleshaving the characteristics described above into the lead body obtainedin the third embodiment described above, that is, the lead body preparedby subjecting a blend composition for a pencil lead containing at leastthe composite flake graphite on which the nanoparticles having thecharacteristics described above are fixed to burning treatment ornon-burning treatment. In this case, the nanoparticles contained in thelead body and the nanoparticles contained in the liquid are completelyindependent from each other, and therefore the same or differentnanoparticles may be used in different amounts. In this case, apreferred content of the nanoparticles is maximum 10% in the pencillead.

Fourth Embodiment

A pencil lead which is the fourth embodiment of the present invention isa pencil lead containing flake graphite which has an ab face having aflatness of at least 2 μm or less and in which an aspect ratio of an aaxis or a b axis in the ab face to a c axis is 5 or more, whereinnanoparticles having a my value of 0.05 to 2 based on 100 of a volumeaverage diameter (my value) of the graphite and having a specificsurface area of 50 to 800 m²/g are adhered to the ab face of thegraphite.

Further, a production process for the pencil lead in the fourthembodiment of the present invention is characterized by bringing flakegraphite which has an ab face having a flatness of at least 2 μm or lessand in which an aspect ratio of an a axis or a b axis in the ab face toa c axis is 5 or more into contact with nanoparticles having a my valueof 0.05 to 2 based on 100 of a volume average diameter (my value) of thegraphite and having a specific surface area of 50 to 800 m²/g, thenfixing the nanoparticles thereon to prepare a composite, forming a leadbody by using the composite graphite and impregnating it with a liquidhaving a refractive index of 1.3 to 1.5 and a viscosity of 7 to 200mm²/s at 25° C.

The fourth embodiment of the present invention is different only interms of using the nanoparticles having a my value of 0.05 to 2 based on100 of a volume average diameter (my value) of the flake graphite andhaving a specific surface area of 50 to 800 m²/g in place of thenanoparticles used in the third embodiment described above which have amy value of 0.05 to 2 based on 100 of a volume average diameter (myvalue) of the flake graphite described above and which have a sphericityof 0.1 to 20 nm and allowing the nanoparticles to be adhered to the abface of the graphite. Also, the nanoparticles used in the secondembodiment are used as well for the nanoparticles. Accordingly, pointsdifferent from the first embodiment and the third embodiment shall bedescribed below in detail, and explanations of the same constitution,production process and the like as the above respective embodimentsshall be omitted.

In the fourth embodiment of the present invention, the lead body isdipped as it is in the liquid having the same characteristics as in thefirst embodiment to the third embodiment described above or subjected todipping treatment and the like under applying pressure (for example, 0.5to 5 MPa) and/or under heating (for example, liquid temperature: 60 to200° C.), whereby obtained is the targeted pencil lead, that is, thepencil lead which contains the graphite having the characteristicsdescribed above and in which the nanoparticles having a my value of 0.05to 2 based on 100 of a volume average diameter (my value) of thegraphite and having a specific surface area of 50 to 800 m²/g areadhered to the ab face of the graphite.

The pencil lead obtained is turned into a pencil lead provided with asuited abrasion characteristic and the like by producing it by theproduction process described above. More preferred is the pencil lead inwhich a dynamic frictional coefficient obtained by dividing an averagevalue (n=10) of total frictional forces in lines drawn by a drawingmethod using a drawing apparatus prescribed in JIS S 6005:2007 by awriting load is 0.191 to 0.218, and this makes it possible to obtain alead by which smoother writing is felt as well in a mechanical pencillead of a type in which a lead is rotated.

Also, when a polished cross section of the pencil lead is observed in anarea of 5 μm×5 μm by means of FE-SEM (S-4700 model, acceleratingvoltage: 5 kV, current value: 10 μA, manufactured by HitachiHigh-Technologies Corporation), 1 to 300 nanoparticles are preferablyobserved. In addition of the nanoparticles in “the more preferred range”described above, 2 to 100 particles are observed, and in additionthereof in “the particularly preferred range”, 5 to 50 particles areobserved.

The above total frictional coefficient and the number of thenanoparticles can be controlled by suitably combining the physicalproperties of the flake graphite used such as a flatness, an aspectratio and a content thereof and a sphericity, a volume average diameter(mv value) and a content (impregnation amount) of the nanoparticles andthe kind of the oil.

In the fourth embodiment of the present invention thus constituted, theflake graphite which has an ab face having a flatness of at least 2 μmor less and in which an aspect ratio of an a axis or a b axis in the abface to a c axis is 5 or more is brought into contact with thenanoparticles having a my value of 0.05 to 2 based on 100 of a volumeaverage diameter (mv value) of the graphite and having a specificsurface area of 50 to 800 m²/g; then the nanoparticles are fixed thereonto prepare a composite; a lead body is formed by using a blendcomposition for a pencil lead containing the above composite graphite,and the lead body for a pencil lead is then impregnated with the liquidhaving the characteristic described above, whereby prepared is a statein which the nanoparticles are penetrated into the pores of a porousbody constituted from the flake graphite having the characteristicsdescribed above (a state in which the nanoparticles are adhered to theab face of the flake graphite). Thus, the properties of the pencil leadhaving a porous structure are different from those of conventional ones.To be specific, the nanoparticles having the characteristics describedabove play a role of a suspension or a bearing, and therefore the leadis improved in lubrication to a larger extent than in a case where thenanoparticles are not added. This allows the pencil lead to be improvedin lubrication to a large extent. Further, since penetration of thenanoparticles into the lead body allows the smooth drawn lines to bringabout diffused reflection, so-called “shine” disappears, and it resultsin making the color dense. In addition thereto, friction between paperand the flake graphite particles and between the flake graphiteparticles themselves are reduced by the action of the flake graphiteitself having the characteristics described above, and the erasabilityis enhanced. Further, since the nanoparticles can evenly be dispersedwithout disturbing orientation of the flake graphite having thecharacteristics described above, an effect thereof as an extender isexerted, and the compression strength is enhanced as well. Also, sincean abrasion amount of the lead is small, an amount of the graphitecontained in the drawn lines is small as well, and the fingers are lessliable to be stained.

Further, in the present invention, prepared is a pencil lead having, inaddition to the action and effect described above, drawn line intensity,writing feeling and a low static•dynamic frictional coefficient whichexceed those of the pencil leads disclosed in patent document 1described above. In particular, even in a case of a pencil lead used fora mechanical pencil of a type in which a lead is rotated wheneverwritten to always carry out writing by a fresh part, obtained are apencil lead which has better and smoother writing feeling and whichprovides drawn lines with higher intensity and a brilliant black colorand a production process for the same (this matter shall be explained infurther details in Examples 19 to 24 and Comparative Examples 23 to 30each described later).

The pencil lead prepared in the fourth embodiment of the presentinvention shall not be restricted to the fourth embodiment describedabove and can be varied in various manners and in the scope of thetechnical concept of the present invention. For example, it may be apencil lead prepared by filling a liquid containing the nanoparticleshaving the characteristics described above into the lead body obtainedin the fourth embodiment described above, that is, the lead bodyprepared by subjecting a blend composition for a pencil lead containingat least the composite flake graphite on which the nanoparticles havingthe characteristics described above are fixed to burning treatment ornon-burning treatment. In this case, the nanoparticles contained in thelead body and the nanoparticles contained in the liquid are completelyindependent from each other, and therefore the same or differentnanoparticles may be used in different amounts. In this case, apreferred content of the nanoparticles is maximum 10% in the pencillead.

Fifth Embodiment

The fifth embodiment and the sixth embodiment of the present inventionare production processes for obtaining pencil leads which can exert thesame action and effect as those of the pencil leads prepared in thefirst embodiment to the fourth embodiment each described above.

The fifth embodiment of the present invention is characterized bydispersing flake graphite which has an ab face having a flatness of atleast 2 μm or less and in which an aspect ratio of an a axis or a b axisin the ab face to a c axis is 5 or more and nanoparticles having a myvalue of 0.05 to 2 based on 100 of a volume average diameter (mv value)of the graphite and having a sphericity of 0.1 to 20 nm in a plasticizeror a solvent, kneading the dispersion and then forming a lead body fromthe kneaded matter.

The fifth embodiment of the present invention uses the flake graphiteand the nanoparticles each used in the first embodiment described aboveand is different in a production process thereof, and the flake graphiteand the nanoparticles each used are the same as in the first embodiment.Points different from the first embodiment shall be described below indetail, and explanations of the same constitution and the like shall beomitted.

In the fifth embodiment of the present invention, plasticizers used forproducing pencil leads can be used, and they include, for example, atleast one of dioctyl phthalate, dibutyl phthalate, tricresyl phosphate,dioctyl adipate, diallyl isophthalate, propylene carbonate, dioctyladipate, diisononyl adipate, trioctyl trimellitate, tricresyl phosphate,tributyl acetylcitrate and the like.

Also, in the fifth embodiment of the present invention, solvents whichcan dissolve the plasticizers described above are preferred as thesolvent used, and to be specific, at least one of alcohols, ketones,esters, ethers, aromatic hydrocarbons, aliphatic hydrocarbons, siloxanesand the like can be used.

In the fifth embodiment of the present invention, the flake graphitehaving the characteristics described above and the nanoparticles havingthe characteristics described above are dispersed in a solvent or aplasticizer or a plasticizer and a solvent and kneaded, and then a blendcomposition for a pencil lead which is the kneaded matter is subjectedto burning treatment or non-burning treatment, whereby a lead body canbe formed.

Apparatuses by which a solvent and a powder can evenly be dispersed,such as a Henschel mixer, a ball mill, a bead mill, a homogenizer,Nanomizer, Hivis Mix can be used for the dispersing method.

A content of the flake graphite having the characteristics describedabove which is used for forming the above lead body for a pencil lead ispreferably 20 to 80%, more preferably 30 to 70% based on the totalamount of the blend composition for a pencil lead. An optimum valuethereof is varied according to the hardness.

If a content of the flake graphite is less than 20% or exceeds 80%, itresults in breakage of balance between the hardness, the writing feelingand the strength, and therefore that is not preferred.

Further, a content of the nanoparticles having the characteristicsdescribed is controlled so that a content of the nanoparticles in thepencil lead obtained by impregnation treatment is preferably 0.001 to5%, more preferably 0.01 to 1% and particularly preferably 0.02 to 0.5%based on the total amount of the blend composition for a pencil lead.

If a content of the above nanoparticles is less than 0.001%, aneffective pore volume thereof is scarcely varied, and a difference froma pencil lead to which the nanoparticles are not added does not appear.On the other hand, if a content of the nanoparticles exceeds 5%, aneffective pore volume thereof is increased, but the pencil lead isbroken in a structure and reduced in strength, and therefore that is notpreferred.

A content of the plasticizer described above is preferably 5 to 50%,more preferably 10 to 30% based on the total amount of the blendcomposition for a pencil lead which is the kneaded matter in terms ofmoldability, dispersibility of the nanoparticles and a quality of thefinished lead.

Also, a content of the solvent described above is preferably 1 to 30%,more preferably 10 to 20% based on the total amount of the blendcomposition for a pencil lead which is the kneaded matter in terms ofmoldability, dispersibility of the nanoparticles, a quality of thefinished lead and a blend amount of the plasticizer.

In the fifth embodiment of the present invention, the flake graphitehaving the characteristics described above and the nanoparticles havingthe characteristics described above are dispersed in a solvent or aplasticizer or a solvent and a plasticizer and kneaded, and then a blendcomposition for a pencil lead which is the kneaded matter is used for apencil lead. The respective components such as an extender, a lubricantother than the above nanoparticles, the flake graphite, the plasticizerand the solvent can suitably selected and used according to the kind ofthe pencil lead and the like.

In a case where the pencil lead is, for example, a burned pencil leadfor a mechanical pencil, at least carbon black and amorphous carbon inaddition to the flake graphite can be contained therein as is the casewith the first embodiment described above. Also, in a case of anon-burned pencil lead, at least oil & fat and waxes can be containedtherein, and in a case of a burned pencil lead, at least an extender anda ceramic binder can be contained therein.

In the fifth embodiment of the present invention, the blend compositionfor a pencil lead which is the kneaded matter described above andcomprises the respective components (the respective components such asthe extender) used for, for example, the burned pencil lead for amechanical pencil, the non-burned pencil lead and the burned pencil leadare kneaded, molded, dried and subjected to burning treatment ornon-burning treatment (drying at low temperature of 50 to 120° C.) undernon-oxidative atmosphere, whereby a lead body can be formed.

Also, in the fifth embodiment of the present invention, for example, ina case of producing the burned pencil lead for a mechanical pencil,preferably in terms of the strength, the intensity and the writingfeeling, (a) 20 to 80% of the flake graphite having the characteristicsdescribed above, (b) 0.01 to 5% of the nanoparticles having thecharacteristics described above, (c) 5 to 50% of the plasticizer and 0to 30% of the solvent and (d) 30 to 60% of the thermoplastic syntheticresin based on the total amount of the blend composition for a pencillead are dispersed and mixed by means of a Henschel mixer and kneaded bymeans of a pressure kneader or a double roll mill, and the kneadedmatter is molded by means of an extrusion molding machine; and themolded matter is then dried at 110 to 250° C. in an electric furnace andsubsequently burned at 800 to 1400° C. for 20 to 40 hours undernon-oxidative atmosphere (under nitrogen gas atmosphere or inert gasatmosphere), whereby a lead body for forming a pencil lead can beformed.

In the fifth embodiment of the present invention, the pencil lead isobtained by the production process described above, and a clearanceformed between the flake graphites by the nanoparticles is preferablyimpregnated with the same liquid as in the first embodiment describedabove which has a refractive index of 1.3 to 1.5 and a viscosity of 7 to200 mm²/s at 25° C. after forming the pencil lead preferably in terms ofthe intensity, the writing feeling and the erasing performance.

The liquid used in the fifth embodiment of the present invention is usedin order to act as a lubricant as well as for the purpose of enhancingthe intensity, and the liquid having a refractive index of 1.3 to 1.5and a viscosity of 7 to 200 mm²/s at 25° C. is used in terms of easinessof penetration into pores and reflectance of light.

In the fifth embodiment of the present invention, the lead body isdipped as it is in the liquid having the characteristics described aboveor subjected to dipping treatment and the like under applying pressure(for example, 0.5 to 5 MPa) and/or under heating (for example, liquidtemperature: 60 to 200° C.), whereby a more preferred pencil lead isobtained.

The pencil lead obtained by the production process in the fifthembodiment (including a sixth embodiment described later) of the presentinvention is turned into a pencil lead having a suited abrasioncharacteristic and the like. More preferred is the pencil lead in whicha total frictional coefficient (dynamic frictional coefficient) obtainedby dividing an average value (n=10) of total frictional forces in linesdrawn by a drawing method using the drawing apparatus prescribed in JISS 6005:2007 by a writing load is 0.191 to 0.218, and this makes itpossible to obtain a lead by which smoother writing is felt as well in amechanical pencil lead of a type in which a lead is rotated.

Also, when a polished cross section of the pencil lead is observed in anarea of 5 μm×5 μm by means of FE-SEM (S-4700 model, acceleratingvoltage: 5 kV, current value: 10 μA, manufactured by HitachiHigh-Technologies Corporation), 1 to 300 nanoparticles are preferablyobserved. In addition of the nanoparticles in “the more preferred range”described above, 2 to 100 particles are observed, and in additionthereof in “the particularly preferred range”, 5 to 50 particles areobserved (the same shall apply to the sixth embodiment described later).

The above total frictional coefficient and the number of thenanoparticles can be controlled by suitably combining the physicalproperties of the flake graphite used such as a flatness, an aspectratio and a content thereof and a sphericity, a volume average diameter(mv value) and a content (impregnation amount) of the nanoparticles andthe kind of the oil.

In the production process in the fifth embodiment of the presentinvention thus constituted, the flake graphite which has an ab facehaving a flatness of at least 2 μm or less and in which an aspect ratioof an a axis or a b axis in the ab face to a c axis is 5 or more and thenanoparticles having a my value of 0.05 to 2 based on 100 of a volumeaverage diameter (mv value) of the graphite and having a sphericity of0.1 to 20 nm are dispersed in a plasticizer or a solvent and kneaded,and then a lead body is formed from the kneaded matter. Then, preferablya clearance formed between the flake graphites by the nanoparticles isimpregnated with the liquid having a refractive index of 1.3 to 1.5 anda viscosity of 7 to 200 mm²/s at 25° C., whereby prepared is a state inwhich the nanoparticles are penetrated into the pores of a porous bodyconstituted from the flake graphite having the characteristics describedabove (a state in which the nanoparticles are brought into contact witha gap between the flake graphites). Thus, the properties of the pencillead having a porous structure are different from those of conventionalones. To be specific, the nanoparticles having the characteristicsdescribed above play a role of a suspension or a bearing, and thereforethe lead is improved in lubrication to a larger extent than in a casewhere the nanoparticles are not added. This allows the pencil lead to beimproved in lubrication to a large extent. Further, since penetration ofthe nanoparticles into the lead body allows the smooth drawn lines tobring about diffused reflection, so-called “shine” disappears, and itresults in making the color dense. In addition thereto, friction betweenpaper and the flake graphite particles and between the flake graphiteparticles themselves is reduced by the action of the flake graphitehaving the characteristics described above, and the erasability isenhanced. Further, since the nanoparticles can be adhered withoutdisturbing orientation of the flake graphite having the characteristicsdescribed above, an effect thereof as an extender is exerted, and thecompression strength is enhanced as well. Also, since an abrasion amountof the lead is small, an amount of the graphite contained in the drawnlines is small as well, and the fingers are less liable to be stained.

Further, in the production process in the fifth embodiment of thepresent invention, prepared is a pencil lead having, in addition to theaction and effect described above, drawn line intensity, writing feelingand a low static•dynamic frictional coefficient which exceed those ofthe pencil leads disclosed in patent document 1 described above. Inparticular, even in a case of a pencil lead used for a mechanical pencilof a type in which a lead is rotated whenever written to always carryout writing by a fresh part, obtained are a pencil lead which has betterand smoother writing feeling and which provides drawn lines with higherintensity and a brilliant black color and a production process for thesame (this matter shall be explained in further details in Examples 25to 31 and Comparative Examples 31 to 37 each described later).

The production process for the pencil lead in the fifth embodiment ofthe present invention shall not be restricted to the fifth embodimentdescribed above and can be varied in various manners and in the scope ofthe technical concept of the present invention. For example, it may be apencil lead prepared by forming the lead body obtained in the fifthembodiment described above and then further filling it with the liquidhaving the characteristics described above which contains thenanoparticles having the characteristics described above. In this case,the nanoparticles contained in the lead body and the nanoparticlescontained in the liquid are completely independent from each other, andtherefore the same or different nanoparticles may be used in differentamounts. In this case, a preferred content of the nanoparticles ismaximum 10% in the pencil lead.

Sixth Embodiment

The sixth embodiment of the present invention is a production processesfor obtaining a pencil lead which can exert the same action and effectas those of the pencil leads prepared in the first embodiment to thefourth embodiment each described above, and it is characterized bydispersing flake graphite which has an ab face having a flatness of atleast 2 μm or less and in which an aspect ratio of an a axis or a b axisin the ab face to a c axis is 5 or more and nanoparticles having a myvalue of 0.05 to 2 based on 100 of a volume average diameter (my value)of the graphite and having a specific surface area of 50 to 800 m²/g ina plasticizer or a solvent, kneading and then forming a lead body fromthe kneaded matter.

The sixth embodiment of the present invention is different in terms ofusing the nanoparticles having a my value of 0.05 to 2 based on 100 of avolume average diameter (my value) of the graphite and having a specificsurface area of 50 to 800 m²/g which are used in the second embodiment(and the fourth embodiment) in place of the nanoparticles used in thefifth embodiment described above. Points different from the fifthembodiment described above shall be described below in detail, andexplanations of the same constitution and the like shall be omitted.

In the production process in the sixth embodiment of the presentinvention, the graphite which has an ab face having a flatness of atleast 2 μm or less and in which an aspect ratio of an a axis or a b axisin the ab face to a c axis is 5 or more is brought into contact with thenanoparticles having a my value of 0.05 to 2 based on 100 of a volumeaverage diameter (my value) of the graphite and having a specificsurface area of 50 to 800 m²/g, and then the nanoparticles having thecharacteristics described above are dispersed, as is the case with thefifth embodiment described above, in a plasticizer or a solvent andkneaded; a lead body is then formed from the kneaded matter. Then,preferably a clearance formed between the flake graphites by thenanoparticles is impregnated with the liquid having a refractive indexof 1.3 to 1.5 and a viscosity of 7 to 200 mm²/s at 25° C., wherebyprepared is a state in which the nanoparticles are penetrated into thepores of a porous constituted from the flake graphite having thecharacteristics described above (a state in which the nanoparticles arebrought into contact with a gap between the flake graphites). Thus, theproperties of the pencil lead having a porous structure are differentfrom those of conventional ones. To be specific, the nanoparticleshaving the characteristics described above play a role of a suspensionor a bearing, and therefore the lead is improved in lubrication to alarger extent than in a case where the nanoparticles are not added. Thisallows the pencil lead to be improved in lubrication to a large extent.Further, since penetration of the nanoparticles into the lead bodyallows the smooth drawn lines to bring about diffused reflection,so-called “shine” disappears, and it results in making the color dense.In addition thereto, friction between paper and the flake graphiteparticles and between the flake graphite particles themselves is reducedby the action of the flake graphite itself having the characteristicsdescribed above, and the erasability is enhanced. Further, since thenanoparticles can evenly be dispersed without disturbing orientation ofthe flake graphite having the characteristics described above, an effectthereof as an extending is exerted, and the compression strength isenhanced as well. Also, since an abrasion amount of the lead is small,an amount of the graphite contained in the drawn lines is small as well,and the fingers are less liable to be stained.

Further, in the sixth embodiment of the present invention, prepared is apencil lead having, in addition to the action and effect describedabove, drawn line intensity, writing feeling and a low static•dynamicfrictional coefficient which exceed those of the pencil leads disclosedin patent document 1 described above. In particular, even in a case of apencil lead used for a mechanical pencil of a type in which a lead isrotated whenever written to always carry out writing by a fresh part,obtained are a pencil lead which has better and smoother writing feelingand which provides drawn lines with higher intensity and a brilliantblack color and a production process for the same (this matter shall beexplained in further details in Examples 32 to 38 and ComparativeExamples 38 to 45 each described later).

The production process for the pencil lead in the sixth embodiment ofthe present invention shall not be restricted to the sixth embodimentdescribed above and can be varied in various manners and in the scope ofthe technical concept of the present invention. For example, it may be apencil lead prepared by forming the lead body obtained in the sixthembodiment described above and then further filling it with the liquidhaving the characteristics described above which contains thenanoparticles having the characteristics described above. In this case,the nanoparticles contained in the lead body and the nanoparticlescontained in the liquid are completely independent from each other, andtherefore the same or different nanoparticles may be used in differentamounts. In this case, a preferred content of the nanoparticles ismaximum 10% in the pencil lead.

EXAMPLES

Next, the present invention shall be explained in further details byevery respective embodiments (the first embodiment to the sixthembodiment) with reference to examples, reference examples andcomparative examples, but the present invention shall not be restrictedto the examples and the like shown below.

Methods for Measuring the Physical Properties of Flake Natural Graphiteand Nanoparticles:

Physical properties such as a flatness, an aspect ratio of the flakenatural graphite and a sphericity and a specific surface area of thenanoparticles which were used in the examples, the reference examplesand the comparative examples were measured by the following measuringmethods.

Measuring Method for Flatness:

Measured (n=10) is the maximum distance between two parallel lines whichare brought into contact with a particle shown in FIG. 1 observed underSEM whose ab face is perpendicular to an observer and which lines wereparallel to line segments connecting long axis end parts of theparticles.

The aspect ratio is calculated from a c axis length measured from FIG. 1and an ab face measured from the observed image in terms of a ratiothereof.

Measuring Method for Sphericity:

Determined is in terms of the maximum value of distances in a radialdirection from the minimum circumscribed circle to the surface of aparticle with respect to ten particles observed on an SEM or TEM image.

Measuring Method for Specific Surface Area:

A BET specific surface area determined by a BET dynamic method is set toa specific surface area of the nanoparticles and measured by means of afull automatic BET specific surface area measuring apparatus(HM-model-1208) manufactured by Mountech Co., Ltd.

Examples 1 to 6, Reference Example 1 and Comparative Examples 1 to 7First Embodiment Example 1

Flake natural graphite A (ab face having a 40 parts by mass flatness of0.2 μm, mv value: 8 μm, thickness of a c axis: 1 μm, aspect ratio: 8)Polyvinyl chloride 40 parts by mass Sodium stearate  1 part by massDioctyl phthalate 19 parts by mass

The materials described above were mixed and dispersed by means of aHenschel mixer and kneaded by means of a pressure kneader or a rollmill, and after molded, the molded matter was dried to remove dioctylphthalate and subjected to burning treatment at 1000° C. for 10 hours innitrogen gas atmosphere, whereby a burned pencil lead body having adiameter of 0.565 mm and a length of 60 mm was produced.

Next, the burned pencil lead body was impregnated (impregnation time:180 minutes, hereinafter the same shall apply) at a pressure of 1 MPawith liquid A (liquid temperature: 100° C., hereinafter the same shallapply) in which nanoparticles A (0.1% by mass) described below weredispersed, and a nanodiamond-containing burned pencil lead was obtained.

Liquid A: dimethyl silicone oil KF 96-30CS (dynamic viscosity: 30 mm²/s,refractive index: 1.401, manufactured by Shin-Etsu Chemical Co., Ltd.)Nanoparticles A: diamond nanoparticles (sphericity: 3 nm, my value: 10nm, manufactured by Sumiseki Materials Co., Ltd.)

Nanoparticles A described above had a my value of 0.125 based on 100 ofa volume average diameter (mv value) of flake natural graphite A.

Example 2

Liquid B: CMC-Na 1 wt % in distilled water (dynamic viscosity: 7 mm²/s,refractive index: 1.345)Nanoparticles A: diamond nanoparticles (sphericity: 3 nm, my value: 10nm, manufactured by Sumiseki Materials Co., Ltd.)

The burned pencil lead body obtained in Example 1 described above wasimpregnated at a pressure of 1 MPa with liquid B in which nanoparticlesA (0.1% by mass) described above were dispersed, and ananodiamond-containing burned pencil lead was obtained.

Example 3

Liquid C: trimethylpentaphenyltrisiloxane (dynamic viscosity: 175 mm²/s,refractive index: 1.580, manufactured by Toray Industries, Inc.)Nanoparticles A: diamond nanoparticles (sphericity: 3 nm, my value: 10nm, manufactured by Sumiseki Materials Co., Ltd.)

The burned pencil lead body obtained in Example 1 described above wasimpregnated at a pressure of 1 MPa with liquid C in which nanoparticlesA (0.1% by mass) described above were dispersed, and ananodiamond-containing burned pencil lead was obtained.

Example 4

Liquid D: dimethyl silicone KF 96L-5cs (dynamic viscosity: 5 mm²/s,refractive index: 1.396, manufactured by Shin-Etsu Chemical Co., Ltd.)Nanoparticles A: diamond nanoparticles (sphericity: 3 nm, my value: 10nm, manufactured by Sumiseki Materials Co., Ltd.)

The burned pencil lead body obtained in Example 1 described above wasimpregnated at a pressure of 1 MPa with liquid D in which nanoparticlesA (0.1% by mass) described above were dispersed, and ananodiamond-containing burned pencil lead was obtained.

Example 5

Liquid E: dimethyl silicone KF 96-500cs (dynamic viscosity: 500 mm²/s,refractive index: 1.403, manufactured by Shin-Etsu Chemical Co., Ltd.)Nanoparticles A: diamond nanoparticles (sphericity: 3 nm, my value: 10nm, manufactured by Sumiseki Materials Co., Ltd.)

The burned pencil lead body obtained in Example 1 described above wasimpregnated at a pressure of 1 MPa with liquid E in which nanoparticlesA (0.1% by mass) described above were dispersed, and ananodiamond-containing burned pencil lead was obtained.

Example 6

Flake natural graphite A (ab face having a 70 parts by mass flatness of0.2 μm, mv value: 8 μm, thickness of a c axis: 1 μm, aspect ratio: 8)Kaolinite clay  5 parts by mass Halloysite clay 15 parts by mass Water30 parts by mass

The materials described above were mixed and dispersed by means of aHenschel mixer, and the mixture was sufficiently kneaded with heating bymeans of a double roll mill until a moisture there was reduced to about18 parts by mass. The kneaded matter obtained was extrusion-molded inthe form of a linear body by means of a dice for extrusion, and then themolded matter was subjected to heat treatment at 120° C. for 20 hours inthe air to remove residual moisture and burned up to 1200° C. in 10hours and at 1200° C. for 1 hour in nitrogen gas atmosphere.

Next, the burned pencil lead body was impregnated with liquid F in whichnanoparticles A (0.1% by mass) described below were dispersed, and awood case pencil lead having a diameter of 2.05 mm was obtained.

Liquid F: Miyoshi Controlled Lard (manufactured by Miyoshi Oil & FatCo., Ltd.)Nanoparticles A: diamond nanoparticles (sphericity: 3 nm, my value: 10nm, manufactured by Sumiseki Materials Co., Ltd.)

The nanoparticles described above had a my value of 0.125 based on 100of a volume average diameter (mv value) of flake natural graphite A.

Reference Example The Nanoparticles were Mixed and Dispersed in theMaterials

Flake natural graphite A (ab face having a  40 parts by mass flatness of0.2 μm, mv value: 8 μm, thickness of a c axis: 1 μm, aspect ratio: 8)Diamond nanoparticles (sphericity: 3 nm, 0.1 part by mass mv value: 10nm, manufactured by Sumiseki Materials Co., Ltd.) Polyvinyl chloride  40parts by mass Sodium stearate   1 part by mass Dioctyl phthalate  19parts by mass

The materials described above were mixed and dispersed by means of aHenschel mixer and kneaded by means of a pressure kneader or a rollmill, and after molded, the molded matter was dried to remove dioctylphthalate and then subjected to burning treatment at 1000° C. for 10hours in nitrogen gas atmosphere, whereby a burned pencil lead bodyhaving a diameter of 0.565 mm and a length of 60 mm was produced.

Next, the burned pencil lead body described above was impregnated at apressure of 1 MPa with liquid A used in Example 1 described above, and ananodiamond-containing burned pencil lead was obtained.

Comparative Example 1 Based on Example 11 of Japanese Patent ApplicationLaid-Open No. 138031/2007

Flake natural graphite (flatness of 3 μm, 49 parts by mass mv value: 10μm, thickness of a c axis: 1 μm, aspect ratio: 10) Diamond nanoparticles(single crystal diamond,  1 part by mass sphericity: 1.5 nm, mv value: 5nm) Polyvinyl chloride 50 parts by mass Sodium stearate  1 part by massDioctyl phthalate 20 parts by mass

The materials described above were mixed and dispersed by means of aHenschel mixer and kneaded by means of a pressure kneader or a doubleroll mill, and the kneaded matter was extrusion-molded in the form of alinear body. Then, the molded matter was subjected to heat treatment inthe air in order to remove the residual plasticizer and solidified(dried), and it was then burned at 1000° C. in nitrogen gas atmosphere.Finally, the burned pencil lead was dipped in α-olefin oligomer(Lipolupe 20, manufactured by Lion Corporation) to obtain a lead HB fora mechanical pencil having a diameter of 0.570 mm.

Comparative Example 2

Liquid A: dimethyl silicone oil KF 96-30CS (dynamic viscosity: 30 mm²/s,refractive index: 1.401, manufactured by Shin-Etsu Chemical Co., Ltd.)Nanoparticles B: diamond nanoparticles (sphericity: 25 nm, my value: 50nm, manufactured by Sumiseki Materials Co., Ltd.)

The burned pencil lead body obtained in Example 1 described above wasimpregnated (impregnation time: 180 minutes) at a pressure of 1 MPa withliquid B (liquid temperature: 100° C.) in which nanoparticles B (0.1% bymass) described above were dispersed, and a nanodiamond-containingburned pencil lead was obtained.

Comparative Example 3

A nanodiamond-containing burned pencil lead was obtained in the samemanner as in Example 1, except that the flake natural graphite A (abface having a flatness of 0.2 μm, my value: 8 μm, thickness of a c axis:1 μm, aspect ratio: 8) used in Example 1 described above was changed tothe same amount of flake natural graphite B (ab face having a flatnessof 3 μm, my value: 10 μm, thickness of a c axis: 1 μm, aspect ratio:10).

Comparative Example 4

A nanodiamond-containing burned pencil lead was obtained in the samemanner as in Example 1, except that the flake natural graphite A (abface having a flatness of 0.2 μm, my value: 8 μm, thickness of a c axis:1 μm, aspect ratio: 8) used in Example 1 described above was changed tothe same amount of flake natural graphite C (ab face having a flatnessof 0.2 μm, my value: 3 μm, thickness of a c axis: 1 μm, aspect ratio:3).

Comparative Example 5

The pencil lead body obtained in Example 1 described above wasimpregnated under pressure, as was the case with Example 1, with liquidA used in Example 1 containing no nanoparticles A, and ananodiamond-containing burned pencil lead was obtained.

Comparative Example 6 Based on Example 11 of Japanese Patent ApplicationLaid-Open No. 2007-138031

Flake natural graphite (flatness of 3 μm, 69 parts by mass mv value: 10μm, thickness of a c axis: 1 μm, aspect ratio: 10) Diamond nanoparticles(single crystal  1 part by mass diamond, sphericity: 1.5 nm, mv value: 5nm) Kaolinite clay 15 parts by mass Halloysite clay 15 parts by massWater 30 parts by mass

The materials described above were mixed and dispersed by means of aHenschel mixer, and the mixture was sufficiently kneaded with heating bymeans of a double roll mill until a moisture there was reduced to about18 parts by mass. The kneaded matter obtained was extrusion-molded inthe form of a linear body by means of a dice for extrusion, and then themolded matter was subjected to heat treatment at 120° C. for 20 hours inthe air to remove residual moisture and burned up to 1200° C. in 10hours and at 1200° C. for 1 hour in nitrogen gas atmosphere.

Next, the burned pencil lead body was impregnated with liquid F (MiyoshiControlled Lard) used in Example 6, and a wood case pencil lead having adiameter of 2.05 mm was obtained.

Comparative Example 7

The pencil lead body obtained in Example 6 described above wasimpregnated, as was the case with Example 6, with liquid F (MiyoshiControlled Lard) used in Example 6 containing no nanoparticles A, and awood case pencil lead having a diameter of 2.05 mm was obtained.

The respective burned pencil leads (pencil leads for a mechanical penciland wood case pencil leads) obtained in Examples 1 to 6, ReferenceExample 1 and Comparative Examples 1 to 7 were used to evaluate bendingstrength, compression strength (N), an abrasion amount (mm), intensity,an erasing rate (%), a frictional coefficient (static and dynamic), ananoparticle number, writing feeling by sensory evaluation, difficultyof stain and initial sliding by the following respective evaluationmethods. The results thereof are shown in the following Table 1.

Measuring Method for Bending Strength:

The pencil leads for a mechanical pencil obtained in Examples 1 to 5,Reference Example and Comparative Examples 1 to 6 were used to measurebending strength thereof by a bending strength test prescribed in JIS S6005:2007 (n=10). Further, the wood case pencil leads obtained inExample 6 and Comparative Examples 6 and 7 were used to measure bendingstrength thereof by a bending strength test prescribed in JIS S6006:2007 (n=10).

Measuring Method for Compression Strength:

The lead was put horizontally on a flat plane and fixed thereon, and itwas subjected to a compression test from upward by means of Tensilon(ORIENTEC RTC-1150A) which was a compression tool having a lateral widthof 2 mm and a longitudinal width of 5 mm to measure breaking strength(n=10).

The compression strength which is an evaluation item is an index showingdifficulty of breaking by a chuck of a pencil lead for a mechanicalpencil, and therefore it was not measured in the wood case pencil leadsobtained in Example 6 and Comparative Examples 6 and 7 and shown by ┌-┘in the evaluation.

Test Method for Abrasion Test:

An abrasion length of the lead in writing at a writing angle of 75°, aload of 300 gf and a writing distance of 5 m was measured (n=10).

Measuring Method for Intensity:

The density is a value obtained by measuring the lines written in theabrasion test by means of a densitometer (sakura DENSITOMETER PDA65)(n=10×4 positions).

Measuring Method for Erasing Rate:

An eraser (EP-105E) was reciprocated five times on the lines written inthe abrasion test, and then an erasing rate of the lines was determined(n=10).

Measuring Method for Frictional Coefficient:

A value (n=10) obtained by dividing an average value of total frictionalforces in the lines drawn by a drawing method using the drawingapparatus prescribed in JIS S 6005:2007 and JIS S 6006:2007 by a writingload was set to “a dynamic frictional coefficient”, and a value obtainedby dividing a maximum value of friction by a writing load was set to “astatic frictional coefficient”.

Measuring Method for Nanoparticle Number:

The number of the nanoparticles was measured by observing a polishedcross section of the respective pencil leads obtained in an area of 5μm×5 μm by means of FE-SEM (S-4700 model, accelerating voltage: 5 kV,current value: 10 μA, manufactured by Hitachi High-TechnologiesCorporation).

Evaluation Methods for Writing Feeling, Difficulty of Stain and InitialSliding:

Ten testers wrote “Mitsubishi pencil” repeatedly on one copy paper forwriting 400 characters to relatively evaluate the following respectiveitems by comparing with an existing product (“SHU” 0.5 mm-HB,manufactured by Mitsubishi Pencil Co., Ltd.) of the company concerned.

The writing feeling was compared with whether or not felt smooth andevaluated according to the following evaluation criteria.

Stain of the finger after writing 400 characters was compared toevaluate the difficulty of stain according to the following evaluationcriteria.

Whether or not each line was started sliding smoothly was compared toevaluate the initial sliding according to the following evaluationcriteria.

Evaluation Criteria (Average Value):

⊚: very good◯: better than the existing productΔ: equal to the existing productX: worse than the existing product

TABLE 1 Lead Break- Bending Frictional Nano- Sensory evaluation diametering strength Compression Abrasion Erasing coefficient particle WritingDifficulty Initial mm load N MPa strength N mm Intensity rate % StaticDynamic number feeling of stain sliding Example 1 0.565 0.756 426.926.16 2.246 0.419 98.13 0.515 0.183 40 ⊚ ⊚ ⊚ Example 2 0.565 0.756 426.926.13 1.732 0.311 95.22 0.620 0.186 42 ⊚ ⊚ ◯ Example 3 0.565 0.754 425.826.17 2.100 0.369 98.11 0.555 0.190 35 ⊚ ◯ ⊚ Example 4 0.565 0.756 426.926.15 2.341 0.415 97.05 0.617 0.184 43 ⊚ ⊚ ◯ Example 5 0.565 0.756 426.926.14 1.811 0.413 94.67 0.526 0.200 33 ◯ ◯ ⊚ Example 6 2.050 6.850 121.5— 1.523 0.409 97.63 0.736 0.211 29 ⊚ ⊚ ◯ Reference 0.565 0.735 415.125.12 2.011 0.448 95.68 0.681 0.191 1000 or ⊚ ⊚ ◯ Example 1 more Com-0.570 0.751 413.1 26.01 1.782 0.432 95.51 0.750 0.200 1000 or ⊚ ⊚ Δparative more Example 1 Com- 0.570 0.755 415.3 26.03 1.606 0.367 95.430.790 0.201 1 ◯ ⊚ X parative Example 2 Com- 0.570 0.743 408.7 25.832.244 0.412 92.38 0.737 0.227 40 ◯ Δ Δ parative Example 3 Com- 0.5700.726 399.3 26.24 3.041 0.413 89.25 0.802 0.216 45 ◯ X X parativeExample 4 Com- 0.570 0.753 414.2 26.17 2.033 0.400 97.17 0.773 0.224 0 ΔΔ Δ parative Example 5 Com- 2.050 6.220 110.3 — 1.623 0.433 95.44 0.6960.209 1000 or ◯ ◯ ⊚ parative more Example 6 Com- 2.050 6.630 117.6 —1.508 0.399 96.85 0.770 0.229 0 Δ Δ Δ parative Example 7

Comments on Examples, Reference Example and Comparative Examples in theFirst Embodiment

As apparent from the results shown in Table 1, it has been found thatthe respective pencil leads for a mechanical pencil obtained in Examples1 to 5 and the wood case pencil lead obtained in Example 6 each fallingin the scope of the present invention are excellent in bending strengthand compression strength and have a satisfactory color forming propertyand satisfactory drawn line intensity and that they have less abrasion,a good erasing property, good initial sliding and good writing feeling(pen touch) and result in being difficult of stain as compared with therespective pencil leads for a mechanical pencil obtained in ComparativeExamples 1 to 5 and Reference Example 1 and the wood case pencil leadsobtained in Comparative Examples 6 and 7 each falling outside the scopeof the present invention.

In contrast with this, to observe individually the comparative examples,Comparative Example 1 is a case where the nanoparticles based on Example11 of Japanese Patent Application Laid-Open No. 2007-138031 fallingoutside the scope of the present invention were used; ComparativeExamples 3 and 4 are cases where the flake graphites falling outside thescope of the present invention were used; Comparative Example 5 is acase where the nanoparticles were not used; and it has been found thatthe targeted pencil leads are not obtained from the above respectivepencil leads for a mechanical pencil. Further, Comparative Example 6provides the wood case pencil lead based on Example 11 of JapanesePatent Application Laid-Open No. 2007-138031; Comparative Example 7provides the wood case pencil lead in which the nanoparticles were notused; and it has been found that the targeted pencil leads are notobtained from the above wood case pencil leads.

Examples 7 to 12, Reference Example 2 and Comparative Examples 8 to 15Second Embodiment Example 7

Flake natural graphite A (ab face having 40 parts by mass a flatness of0.2 μm, mv value: 8 μm, thickness of a c axis: 1 μm, aspect ratio: 8)Polyvinyl chloride 40 parts by mass Sodium stearate  1 part by massDioctyl phthalate 19 parts by mass

The materials described above were mixed and dispersed by means of aHenschel mixer and kneaded by means of a pressure kneader and a rollmill, and after molded, the molded matter was dried to remove dioctylphthalate and subjected to burning treatment at 1000° C. for 10 hours innitrogen gas atmosphere, whereby a burned pencil lead body having adiameter of 0.565 mm and a length of 60 mm was produced.

Next, the burned pencil lead body was impregnated (impregnation time:180 minutes, hereinafter the same shall apply) at a pressure of 1 MPawith liquid A (liquid temperature: 100° C., hereinafter the same shallapply) in which nanoparticles A (0.1% by mass) described below weredispersed, and a nanodiamond-containing burned pencil lead was obtained.

Liquid A: dimethyl silicone oil KF 96-30CS (dynamic viscosity: 30 mm²/s,refractive index: 1.401, manufactured by Shin-Etsu Chemical Co., Ltd.)Nanoparticles A: diamond nanoparticles (specific surface area: 450 m²/g,my value: 10 nm, manufactured by Sumiseki Materials Co., Ltd.)

The nanoparticles A had a my value of 0.125 based on 100 of a volumeaverage diameter (mv value) of flake natural graphite A.

Example 8

Liquid B: CMC-Na 1 wt % in distilled water (dynamic viscosity: 7 mm²/s,refractive index: 1.345)Nanoparticles A: diamond nanoparticles (specific surface area of 450m²/g, my value: 10 nm, manufactured by Sumiseki Materials Co., Ltd.)

The burned pencil lead body obtained in Example 7 described above wasimpregnated at a pressure of 1 MPa with liquid B in which nanoparticlesA (0.1% by mass) described above were dispersed, and ananodiamond-containing burned pencil lead was obtained.

Example 9

Liquid C: trimethylpentaphenyltrisiloxane (dynamic viscosity: 175 mm²/s,refractive index: 1.580, manufactured by Toray Industries, Inc.)Nanoparticles A: diamond nanoparticles (specific surface area of 450m²/g, my value: 10 nm, manufactured by Sumiseki Materials Co., Ltd.)

The burned pencil lead body obtained in Example 7 described above wasimpregnated at a pressure of 1 MPa with liquid C in which nanoparticlesA (0.1% by mass) described above were dispersed, and ananodiamond-containing burned pencil lead was obtained.

Example 10

Liquid D: dimethyl silicone KF 96L-5cs (dynamic viscosity: 5 mm²/s,refractive index: 1.396, manufactured by Shin-Etsu Chemical Co., Ltd.)Nanoparticles A: diamond nanoparticles (specific surface area of 450m²/g, my value: 10 nm, manufactured by Sumiseki Materials Co., Ltd.)

The burned pencil lead body obtained in Example 7 described above wasimpregnated at a pressure of 1 MPa with liquid D in which nanoparticlesA (0.1% by mass) described above were dispersed, and ananodiamond-containing burned pencil lead was obtained.

Example 11

Liquid E: dimethyl silicone KF 96-500cs (dynamic viscosity: 500 mm²/s,refractive index: 1.403, manufactured by Shin-Etsu Chemical Co., Ltd.)Nanoparticles A: diamond nanoparticles (specific surface area: 450 m²/g,my value: 10 nm, manufactured by Sumiseki Materials Co., Ltd.)

The burned pencil lead body obtained in Example 7 described above wasimpregnated at a pressure of 1 MPa with liquid E in which nanoparticlesA (0.1% by mass) described above were dispersed, and ananodiamond-containing burned pencil lead was obtained.

Example 12

Flake natural graphite A (ab face having 70 parts by mass a flatness of0.2 μm, mv value: 8 μm, thickness of a c axis: 1 μm, aspect ratio: 8)Kaolinite clay 15 parts by mass Halloysite clay 15 parts by mass Water30 parts by mass

The materials described above were mixed and dispersed by means of aHenschel mixer, and the mixture was sufficiently kneaded with heating bymeans of a double roll mill until a moisture there was reduced to about18 parts by mass. The kneaded matter obtained was extrusion-molded inthe form of a linear body by means of a dice for extrusion, and then themolded matter was subjected to heat treatment at 120° C. for 20 hours inthe air to remove residual moisture and burned up to 1200° C. in 10hours and at 1200° C. for 1 hour in nitrogen gas atmosphere.

Next, the burned pencil lead body was impregnated with liquid F in whichnanoparticles A (0.1% by mass) described below were dispersed, and awood case pencil lead having a diameter of 2.05 mm was obtained.

Liquid F: Miyoshi Controlled Lard (manufactured by Miyoshi Oil & FatCo., Ltd.)Nanoparticles A: diamond nanoparticles (specific surface area: 450 m²/g,my value: 10 nm, manufactured by Sumiseki Materials Co., Ltd.)

The nanoparticles described above had a my value of 0.125 based on 100of a volume average diameter (mv value) of flake natural graphite A.

Reference Example 2 Nanoparticles A were Mixed and Dispersed in theMaterials

Flake natural graphite A (ab face having  40 parts by mass a flatness of0.2 μm, mv value: 8 μm, thickness of a c axis: 1 μm, aspect ratio: 8)Diamond nanoparticles (specific surface 0.1 part by mass area: 450 m²/g,mv value: 10 nm, manufactured by Sumiseki Materials Co., Ltd.) Polyvinylchloride  40 parts by mass Sodium stearate   1 part by mass Dioctylphthalate  19 parts by mass

The materials described above were mixed and dispersed by means of aHenschel mixer and kneaded by means of a pressure kneader and a rollmill, and after molded, the molded matter was dried to remove dioctylphthalate and then subjected to burning treatment at 1000° C. for 10hours in nitrogen gas atmosphere, whereby a burned pencil lead bodyhaving a diameter of 0.565 mm and a length of 60 mm was produced.

Next, the burned pencil lead body described above was impregnated at apressure of 1 MPa with liquid A used in Example 7 described above, and ananodiamond-containing burned pencil lead was obtained.

Comparative Example 8 Based on Example 11 of Japanese Patent ApplicationLaid-Open No. 2007-138031

Flake natural graphite (flatness of 3 μm, 49 parts by mass mv value: 10μm, thickness of a c axis: 1 μm, an aspect ratio of 10) Diamondnanoparticles (cluster diamond,  1 part by mass specific surface area:820 m²/g, mv value: 5 nm) Polyvinyl chloride 50 parts by mass Sodiumstearate  1 part by mass Dioctyl phthalate 20 parts by mass

The materials described above were mixed and dispersed by means of aHenschel mixer and kneaded by means of a pressure kneader and a doubleroll mill, and the kneaded matter was extrusion-molded in the form of alinear body. Then, the molded matter was subjected to heat treatment inthe air in order to remove the residual plasticizer and solidified(dried), and it was then burned at 1000° C. in nitrogen gas atmosphere.Finally, the burned pencil lead body was dipped in an α-olefin oligomer(Lipolupe 20, manufactured by Lion Corporation) to obtain a lead HB fora mechanical pencil having a diameter of 0.570 mm.

Comparative Example 9

Liquid A: dimethyl silicone oil KF 96-30CS (dynamic viscosity: 30 mm²/s,refractive index: 1.401, manufactured by Shin-Etsu Chemical Co., Ltd.)Nanoparticles B: diamond nanoparticles (single crystal diamond, specificsurface area of 18.54 m²/g, my value: 100 nm, manufactured by SumisekiMaterials Co., Ltd.)

The burned pencil lead body obtained in Example 7 described above wasimpregnated (impregnation time: 180 minutes) at a pressure of 1 MPa withliquid B (liquid temperature: 100° C.) in which nanoparticles B (0.1% bymass) described above were dispersed, and a nanodiamond-containingburned pencil lead was obtained.

Comparative Example 10

Liquid A: dimethyl silicone oil KF 96-30CS (dynamic viscosity: 30 mm²/s,refractive index: 1.401, manufactured by Shin-Etsu Chemical Co., Ltd.)Nanoparticles C: diamond nanoparticles (cluster diamond, specificsurface area: 848 m²/g, my value: 20 nm, manufactured by SumisekiMaterials Co., Ltd.)

The burned pencil lead body obtained in Example 7 described above wasimpregnated at a pressure of 1 MPa with liquid B (liquid temperature:100° C.) in which nanoparticles B (0.1% by mass) described above weredispersed, and a nanodiamond-containing burned pencil lead was obtained.

Comparative Example 11

A nanodiamond-containing burned pencil lead was obtained in the samemanner as in Example 7, except that flake natural graphite A (ab facehaving a flatness of 0.2 μm, my value: 8 μm, thickness of a c axis: 1μm, aspect ratio: 8) used in Example 7 described above was changed tothe same amount of flake natural graphite B (ab face having a flatnessof 3 μm, my value: 10 μm, thickness of a c axis: 1 μm, aspect ratio:10).

Comparative Example 12

A nanodiamond-containing burned pencil lead was obtained in the samemanner as in Example 7, except that flake natural graphite A (ab facehaving a flatness of 0.2 μm, my value: 8 μm, thickness of a c axis: 1μm, aspect ratio: 8) used in Example 7 described above was changed tothe same amount of flake natural graphite C (ab face having a flatnessof 0.2 μm, my value: 3 μm, thickness of a c axis: 1 μm, aspect ratio:3).

Comparative Example 13

The pencil lead body obtained in Example 7 described above wasimpregnated under pressure, as was the case with Example 7, with liquidA used in Example 7 containing no nanoparticles A, and ananodiamond-containing burned pencil lead was obtained.

Comparative Example 14 Based on Example 11 of Japanese PatentApplication Laid-Open No. 2007-138031

Flake natural graphite A (ab face having 69 parts by mass a flatness of0.2 μm, mv value: 8 μm, thickness of a c axis: 1 μm, aspect ratio: 8)Diamond nanoparticles (cluster diamond,  1 part by mass specific surfacearea of 820 m²/g, mv value: 5 nm) Kaolinite clay 15 parts by massHalloysite clay 15 parts by mass Water 30 parts by mass

The materials described above were mixed and dispersed by means of aHenschel mixer, and the mixture was sufficiently kneaded with heating bymeans of a double roll mill until a moisture there was reduced to about18 parts by mass. The kneaded matter obtained was extrusion-molded inthe form of a linear body by means of a dice for extrusion, and then themolded matter was subjected to heat treatment at 120° C. for 20 hours inthe air to remove residual moisture and burned up to 1200° C. in 10hours and at 1200° C. for 1 hour in nitrogen gas atmosphere.

Next, the burned pencil lead body was impregnated with liquid F (MiyoshiControlled Lard) used in Example 12, and a wood case pencil lead havinga diameter of 2.05 mm was obtained.

Comparative Example 15

The pencil lead body obtained in Example 12 described above wasimpregnated, as was the case with Example 12, with liquid F (MiyoshiControlled Lard) used in Example 12 containing no nanoparticles A, and awood case pencil lead having a diameter of 2.05 mm was obtained.

The respective burned pencil leads (pencil leads for a mechanical penciland wood case pencil leads) obtained in Examples 7 to 12, ReferenceExample 2 and Comparative Examples 8 to 15 were used to evaluate bendingstrength, compression strength (N), an abrasion amount (mm), intensity,an erasing rate (%), a frictional coefficient (static and dynamic), ananoparticle number, writing feeling by sensory evaluation, difficultyof stain and initial sliding by the respective evaluation methodsdescribed above. The results thereof are shown in the following Table 2.

TABLE 2 Lead Bending Com- Sensory Nano- Sensory evaluation diameterBreaking strength pression Abrasion Erasing coefficient particle WritingDifficulty Initial mm load N MPa strength N mm Intensity rate % StaticDynamic number feeling of stain sliding Example 7 0.565 0.756 426.926.15 2.246 0.419 98.06 0.504 0.179 40 ⊚ ⊚ ⊚ Example 8 0.565 0.754 425.826.15 1.740 0.323 95.38 0.612 0.199 42 ⊚ ⊚ ◯ Example 9 0.565 0.754 425.826.15 2.161 0.366 98.24 0.560 0.188 35 ⊚ ◯ ⊚ Example 0.565 0.754 425.826.15 2.322 0.416 97.09 0.605 0.181 43 ⊚ ⊚ ◯ 10 Example 0.565 0.754425.8 26.15 1.824 0.400 94.48 0.530 0.198 33 ◯ ◯ ⊚ 11 Example 2.0506.840 121.3 — 1.535 0.417 97.28 0.729 0.201 26 ⊚ ⊚ ◯ 12 Reference 0.5650.735 415.1 25.12 2.011 0.448 95.68 0.681 0.191 1000 or ⊚ ⊚ ◯ Example 2more Com- 0.570 0.751 413.1 26.01 1.782 0.432 95.51 0.750 0.200 1000 or⊚ ⊚ Δ parative more Example 8 Com- 0.570 0.690 379.5 26.51 2.545 0.39794.07 0.760 0.223 1 ◯ ⊚ Δ parative Example 9 Com- 0.570 0.652 358.622.15 1.350 0.298 88.80 0.781 0.231 10 Δ Δ Δ parative Example 10 Com-0.570 0.746 410.3 25.85 2.253 0.402 92.13 0.789 0.225 40 ◯ Δ Δ parativeExample 11 Com- 0.570 0.728 400.4 26.23 3.049 0.405 89.21 0.822 0.211 45◯ X X parative Example 12 Com- 0.570 0.755 415.3 26.15 1.999 0.401 97.160.776 0.221 0 Δ Δ Δ parative Example 13 Com- 2.050 6.280 111.4 — 1.6440.443 94.40 0.696 0.203 1000 or ◯ ◯ ⊚ parative more Example 14 Com-2.050 6.630 117.6 — 1.508 0.399 96.85 0.770 0.229 0 Δ Δ Δ parativeExample 15

Comments on Examples, Reference Example and Comparative Examples in theSecond Embodiment

As apparent from the results shown in Table 2, it has been found thatthe respective pencil leads for a mechanical pencil obtained in Examples7 to 11 and the wood case pencil lead obtained in Example 12 eachfalling in the scope of the present invention are excellent in bendingstrength and compression strength and have a satisfactory color formingproperty and satisfactory drawn line intensity and that they have lessabrasion, a good erasing property, good initial sliding and good writingfeeling (pen touch) and result in being difficult of stain as comparedwith the respective pencil leads for a mechanical pencil obtained inComparative Examples 8 to 13 and Reference Example 2 and the wood casepencil leads obtained in Comparative Examples 14 and 15 each fallingoutside the scope of the present invention.

In contrast with this, to observe individually the comparative examples,Comparative Example 8 is based on Example 11 of Japanese PatentApplication Laid-Open No. 138031/2007; Comparative Examples 9 and 10 arecases where the nanoparticles falling outside the scope of the presentinvention were used; Comparative Examples 11 and 12 are cases where theflake graphites falling outside the scope of the present invention wereused; Comparative Example 13 is a case where the nanoparticles were notused; and it has been found that the targeted pencil leads are notobtained from the above respective pencil leads for a mechanical pencil.Further, Comparative Example 14 provides the wood case pencil lead basedon Example 11 of Japanese Patent Application Laid-Open No. 138031/2007;Comparative Example 15 provides the wood case pencil lead in which thenanoparticles were not used; and it has been found that the targetedpencil leads are not obtained from the above wood case pencil leads.

Examples 13 to 18 and Comparative Examples 16 to 22 Third EmbodimentExample 13

Flake natural graphite A (ab face having  40 parts by mass a flatness of0.2 μm, mv value: 8 μm, thickness of a c axis: 1 μm, aspect ratio: 8)Nanoparticles A: diamond nanoparticles 0.4 part by mass (sphericity: 10nm, mv value: 50 nm, manufactured by Sumiseki Materials Co., Ltd.)Polyvinyl chloride  40 parts by mass Sodium stearate   1 part by massDioctyl phthalate  19 parts by mass

The flake graphite and nanoparticles A each described above were putinto a Henschel mixer rotated at a high speed (2000 rpm, hereinafter thesame shall apply) to produce the flake graphite to which the diamondnanoparticles were adhered (time for adhesion: 20 minutes, hereinafterthe same shall apply). Then, the remaining materials were put into theHenschel mixer, mixed and dispersed, and the mixture was kneaded bymeans of a pressure kneader and a roll mill. After molded, the moldedmatter was then dried to remove dioctyl phthalate and subjected toburning treatment at 1000° C. for 10 hours in nitrogen gas atmosphere,whereby a burned pencil lead body having a diameter of 0.565 mm and alength of 60 mm was produced.

Next, the burned pencil lead body described above was impregnated(impregnation time: 180 minutes, hereinafter the same shall apply) at apressure of 1 MPa with liquid A (liquid temperature: 100° C.,hereinafter the same shall apply) described below, and ananodiamond-containing burned pencil lead was obtained.

Liquid A: dimethyl silicone oil KF 96-30CS (dynamic viscosity: 30 mm²/s,refractive index: 1.401, manufactured by Shin-Etsu Chemical Co., Ltd.)

Nanoparticles A described above had a my value of 0.125 based on 100 ofa volume average diameter (mv value) of flake natural graphite A.

Example 14

Liquid B: CMC-Na 1 wt % in distilled water (dynamic viscosity: 7 mm²/s,refractive index: 1.345)

The burned pencil lead body obtained in Example 13 was impregnated at apressure of 1 MPa with liquid B described above to obtain ananodiamond-containing burned pencil lead.

Example 15

Liquid C: trimethylpentaphenyltrisiloxane (dynamic viscosity: 175 mm²/s,refractive index: 1.580, manufactured by Toray Industries, Inc.)

The burned pencil lead body obtained in Example 13 was impregnated at apressure of 1 MPa with liquid C described above to obtain ananodiamond-containing burned pencil lead.

Example 16

Liquid D: dimethyl silicone KF 96L-5cs (dynamic viscosity: 5 mm²/s,refractive index: 1.396, manufactured by Shin-Etsu Chemical Co., Ltd.)

The burned pencil lead body obtained in Example 13 was impregnated at apressure of 1 MPa with liquid D described above to obtain ananodiamond-containing burned pencil lead.

Example 17

Liquid E: dimethyl silicone KF 96-500cs (dynamic viscosity: 500 mm²/s,refractive index: 1.403, manufactured by Shin-Etsu Chemical Co., Ltd.)

The burned pencil lead body obtained in Example 13 was impregnated at apressure of 1 MPa with liquid E described above to obtain ananodiamond-containing burned pencil lead.

Example 18

Flake natural graphite A (ab face having a flatness 70 parts by mass of0.2 μm, mv value: 8 μm, thickness of a c axis: 1 μm, aspect ratio: 8)Nanoparticles A: diamond nanoparticles (sphericity: 0.4 part by mass  10nm, mv value: 50 nm, manufactured by Sumiseki Materials Co., Ltd.)Kaolinite clay 15 parts by mass Halloysite clay 15 parts by mass Water30 parts by mass

The materials described above were mixed and dispersed by means of aHenschel mixer, and the mixture was sufficiently kneaded with heating bymeans of a double roll mill until a moisture there was reduced to about18 parts by mass. The kneaded matter obtained was extrusion-molded inthe form of a linear body by means of a dice for extrusion, and then themolded matter was subjected to heat treatment at 120° C. for 20 hours inthe air to remove residual moisture and burned up to 1200° C. in 10hours and at 1200° C. for 1 hour in nitrogen gas atmosphere.

Next, the burned pencil lead body was impregnated with a liquid Fdescribed below, and a wood case pencil lead having a diameter of 2.05mm was obtained.

Liquid F: Miyoshi Controlled Lard (manufactured by Miyoshi Oil & FatCo., Ltd.)

The nanoparticles described above had a my value of 0.125 based on 100of a volume average diameter (mv value) of flake natural graphite A.

Comparative Example 16 Based on Example 11 of Japanese PatentApplication Laid-Open No. 138031/2007

Flake natural graphite (flatness of 3 μm, mv value: 49 parts by mass 10μm, thickness of a c axis: 1 μm, aspect ratio: 10) Diamond nanoparticles(single crystal diamond,  1 part by mass sphericity: 1.5 nm, mv value: 5nm) Polyvinyl chloride 50 parts by mass Sodium stearate  1 part by massDioctyl phthalate 20 parts by mass

The materials described above were mixed and dispersed by means of aHenschel mixer and kneaded by means of a pressure kneader and a doubleroll mill, and the kneaded matter was extrusion-molded in the form of alinear body. Then, the molded matter was subjected to heat treatment inthe air in order to remove the residual plasticizer and solidified(dried), and it was then burned at 1000° C. in nitrogen gas atmosphere.Finally, the burned pencil lead was dipped in an α-olefin oligomer(Lipolupe 20, manufactured by Lion Corporation) to obtain a lead HB fora mechanical pencil having a diameter of 0.570 mm.

Comparative Example 17

A nanodiamond-containing burned pencil lead was obtained in the samemanner as in Example 13, except that nanoparticles A: diamondnanoparticles (sphericity: 10 nm, my value: 50 nm, manufactured bySumiseki Materials Co., Ltd.) used in Example 13 described above waschanged to the same amount of nanoparticles B: diamond nanoparticles(sphericity: 25 nm, my value: 50 nm, manufactured by Sumiseki MaterialsCo., Ltd.).

Comparative Example 18

A nanodiamond-containing burned pencil lead was obtained in the samemanner as in Example 13, except that flake natural graphite A (ab facehaving a flatness of 0.2 μm, my value: 8 μm, thickness of a c axis: 1μm, aspect ratio: 8) used in Example 13 described above was changed tothe same amount of flake natural graphite B (ab face having a flatnessof 3 μm, my value: 10 μm, thickness of a c axis: 1 μm, aspect ratio:10).

Comparative Example 19

A nanodiamond-containing burned pencil lead was obtained in the samemanner as in Example 13, except that the flake natural graphite A (abface having a flatness of 0.2 μm, my value: 8 μm, thickness of a c axis:1 μm, aspect ratio: 8) used in Example 13 described above was changed tothe same amount of flake natural graphite C (ab face having a flatnessof 0.2 μm, my value: 3 μm, thickness of a c axis: 1 μm, aspect ratio:3).

Comparative Example 20

A pencil lead body obtained in the same composition as in Example 13described above except containing no nanoparticles A was impregnatedunder pressure, as was the case with Example 13, with liquid A used inExample 13, and a nanodiamond-non-containing burned pencil lead wasobtained.

Comparative Example 21 Based on Example 11 of Japanese PatentApplication Laid-Open No 138031/2007

Flake natural graphite (an ab face having a flatness 69 parts by mass of3 μm, mv value: 10 μm, thickness of a c axis: 1 μm, an aspect ratio: 10)Diamond nanoparticles (single crystal diamond,  1 part by masssphericity: 1.5 nm, mv value: 5 nm) Kaolinite clay 15 parts by massHalloysite clay 15 parts by mass Water 30 parts by mass

The materials described above were mixed and dispersed by means of aHenschel mixer, and the mixture was sufficiently kneaded with heating bymeans of a double roll mill until a moisture there was reduced to about18 parts by mass. The kneaded matter obtained was extrusion-molded inthe form of a linear body by means of a dice for extrusion, and then themolded matter was subjected to heat treatment at 120° C. for 20 hours inthe air to remove residual moisture and burned up to 1200° C. in 10hours and at 1200° C. for 1 hour in nitrogen gas atmosphere.

Next, the burned pencil lead body was impregnated with liquid F (MiyoshiControlled Lard) used in Example 18, and a wood case pencil lead havinga diameter of 2.05 mm was obtained.

Comparative Example 22

The pencil lead body obtained in the same composition as in Example 18described above except containing no nanoparticles A was impregnated, aswas the case with Example 18, with liquid F (Miyoshi Controlled Lard)used in Example 18, and a wood case pencil lead having a diameter of2.05 mm was obtained.

The respective burned pencil leads (pencil leads for a mechanical penciland wood case pencil leads) obtained in Examples 13 to 18 andComparative Examples 16 to 22 were used to evaluate bending strength,compression strength (N), an abrasion amount (mm), intensity, an erasingrate (%), a frictional coefficient (static and dynamic), a nanoparticlenumber, writing feeling by sensory evaluation, difficulty of stain andinitial sliding by the respective evaluation methods described above.The results thereof are shown in the following Table 3.

TABLE 3 Lead Break- Bending Com- Frictional Nano- Sensory evaluationdiameter ing strength pression Abrasion Erasing coefficient particleWriting Difficulty Initial mm load N MPa strength N mm Intensity rate %Static Dynamic number feeling of stain sliding Example 13 0.565 0.754425.8 26.17 2.745 0.511 98.01 0.505 0.183 17 ⊚ ⊚ ⊚ Example 14 0.5650.757 427.5 26.16 2.119 0.394 95.34 0.604 0.204 17 ⊚ ⊚ ◯ Example 150.565 0.755 426.4 26.16 2.633 0.442 98.23 0.570 0.194 17 ⊚ ◯ ⊚ Example16 0.565 0.753 425.3 26.16 2.810 0.481 97.12 0.627 0.175 17 ⊚ ⊚ ◯Example 17 0.565 0.752 424.7 26.16 2.211 0.489 94.64 0.529 0.188 17 ◯ ◯⊚ Example 18 2.050 6.650 117.9 — 1.545 0.413 96.97 0.699 0.200 13 ⊚ ⊚ ⊚Comparative 0.570 0.751 413.1 26.01 1.782 0.432 95.51 0.750 0.200 1000or ⊚ ⊚ Δ Example 16 more Comparative 0.570 0.758 416.9 26.48 1.973 0.47295.63 0.794 0.213 18 ◯ ⊚ X Example 17 Comparative 0.570 0.744 409.225.79 2.699 0.494 92.20 0.754 0.236 17 ◯ Δ Δ Example 18 Comparative0.570 0.726 399.3 26.24 3.691 0.482 89.11 0.836 0.224 17 ◯ X X Example19 Comparative 0.570 0.758 416.9 26.23 2.534 0.496 97.00 0.772 0.218 0 ΔΔ Δ Example 20 Comparative 2.050 6.350 112.6 — 1.604 0.419 95.92 0.7540.217 1000 or ◯ ◯ ⊚ Example 21 more Comparative 2.050 6.630 117.6 —1.508 0.399 96.85 0.770 0.229 0 Δ Δ Δ Example 22

Comments on Examples and Comparative Examples in the Third Embodiment

As apparent from the results shown in Table 3, it has been found thatthe respective pencil leads for a mechanical pencil obtained in Examples13 to 17 and the wood case pencil lead obtained in Example 18 eachfalling in the scope of the present invention are excellent in bendingstrength and compression strength and have a satisfactory color formingproperty and satisfactory drawn line intensity and that they have lessabrasion, a good erasing property, good initial sliding and good writingfeeling (pen touch) and result in being difficult of stain as comparedwith the respective pencil leads for a mechanical pencil obtained inComparative Examples 16 to 20 and the wood case pencil leads obtained inComparative Examples 21 and 22 each falling outside the scope of thepresent invention.

In contrast with this, to observe individually the comparative examples,Comparative Example 16 is based on Example 11 of Japanese PatentApplication Laid-Open No. 138031/2007; Comparative Example 17 is a casewhere the nanoparticles falling outside the scope of the presentinvention were used; Comparative Examples 18 and 19 are cases where theflake graphites falling outside the scope of the present invention wereused; Comparative Example 20 is a case where the pencil lead wasproduced without adhering nanoparticles A in blending the materials inExample 13 described above; and it has been found that the targetedpencil leads of the present invention are not obtained from the abovepencil leads. Further, Comparative Example 21 provides the wood casepencil lead based on Example 11 of Japanese Patent Application Laid-OpenNo. 138031/2007; Comparative Example 22 provides the wood case pencillead in which the nanoparticles were not used; and it has been foundthat the targeted pencil leads are not obtained from the above wood casepencil leads.

Examples 19 to 24 and Comparative Examples 23 to 30 Fourth EmbodimentExample 19

Flake natural graphite A (ab face having a flatness 40 parts by mass of0.2 μm, mv value: 8 μm, thickness of a c axis: 1 μm, aspect ratio: 8)Nanoparticles A: diamond nanoparticles (specific 0.4 part by masssurface area of 208 m²/g, mv value: 50 nm, manufactured by SumisekiMaterials Co., Ltd.) Polyvinyl chloride 40 parts by mass Sodium stearate1 part by mass Dioctyl phthalate 19 parts by mass

The flake graphite and nanoparticles A each described above were putinto a Henschel mixer rotated at a high speed (2000 rpm, hereinafter thesame shall apply) to produce flake graphite to which the diamondnanoparticles were adhered (time for adhesion: 20 minutes, hereinafterthe same shall apply). Then, the remaining materials were put into theHenschel mixer, mixed and dispersed, and the mixture was kneaded bymeans of a pressure kneader and a roll mill. After molded, the moldedmatter was dried to remove dioctyl phthalate and subjected to burningtreatment at 1000° C. for 10 hours in nitrogen gas atmosphere, whereby aburned pencil lead body having a diameter of 0.565 mm and a length of 60mm was produced.

Next, the burned pencil lead body described above was impregnated(impregnation time: 180 minutes, hereinafter the same shall apply) at apressure of 1 MPa with liquid A (liquid temperature: 100° C.,hereinafter the same shall apply) described below, and ananodiamond-containing burned pencil lead was obtained.

Liquid A: dimethyl silicone oil KF 96-30CS (dynamic viscosity: 30 mm²/s,refractive index: 1.401, manufactured by Shin-Etsu Chemical Co., Ltd.)

Nanoparticles A described above had a my value of 0.125 based on 100 ofa volume average diameter (mv value) of flake natural graphite A.

Example 20

Liquid B: CMC-Na 1 wt % in distilled water (dynamic viscosity: 7 mm²/s,refractive index: 1.345)

The burned pencil lead body obtained in Example 19 was impregnated at apressure of 1 MPa with liquid B described above to obtain ananodiamond-containing burned pencil lead.

Example 21

Liquid C: trimethylpentaphenyltrisiloxane (dynamic viscosity: 175 mm²/s,refractive index: 1.580, manufactured by Toray Industries, Inc.)

The burned pencil lead body obtained in Example 19 was impregnated at apressure of 1 MPa with liquid C described above to obtain ananodiamond-containing burned pencil lead.

Example 22

Liquid D: dimethyl silicone: KF 96L-5cs (dynamic viscosity: 5 mm²/s,refractive index: 1.396, manufactured by Shin-Etsu Chemical Co., Ltd.)

The burned pencil lead body obtained in Example 19 was impregnated at apressure of 1 MPa with liquid D described above to obtain ananodiamond-containing burned pencil lead.

Example 23

Liquid E: dimethyl silicone: KF 96-500cs (dynamic viscosity: 500 mm²/s,refractive index: 1.403, manufactured by Shin-Etsu Chemical Co., Ltd.)

The burned pencil lead body obtained in Example 19 was impregnated at apressure of 1 MPa with liquid E described above to obtain ananodiamond-containing burned pencil lead.

Example 24

Flake natural graphite A (ab face having a flatness 70 parts by mass of0.2 μm, mv value: 8 μm, thickness of a c axis: 1 μm, aspect ratio: 8)Nanoparticles A: diamond nanoparticles (specific 0.4 part by masssurface area of 208 m²/g, mv value: 50 nm, manufactured by SumisekiMaterials Co., Ltd.) Kaolinite clay 15 parts by mass Halloysite clay 15parts by mass Water 30 parts by mass

The materials described above were mixed and dispersed by means of aHenschel mixer, and the mixture was sufficiently kneaded with heating bymeans of a double roll mill until a moisture there was reduced to about18 parts by mass. The kneaded matter obtained was extrusion-molded inthe form of a linear body by means of a dice for extrusion, and then themolded matter was subjected to heat treatment at 120° C. for 20 hours inthe air to remove residual moisture and burned up to 1200° C. in 10hours and at 1200° C. for 1 hour in nitrogen gas atmosphere.

Next, the burned pencil lead body was impregnated with liquid Fdescribed below to obtain a wood case pencil lead having a diameter of2.05 mm.

Liquid F: Miyoshi Controlled Lard (manufactured by Miyoshi Oil & FatCo., Ltd.)

The nanoparticles described above had a my value of 0.125 based on 100of a volume average diameter (mv value) of flake natural graphite A.

Comparative Example 23 Based on Example 11 of Japanese PatentApplication Laid-Open No. 138031/2007

Flake natural graphite (flatness of 3 μm, mv value: 49 parts by mass 10μm, thickness of a c axis: 1 μm, an aspect ratio: 10) Diamondnanoparticles (cluster diamond, specific  1 part by mass surface area of820 m²/g, mv value: 5 nm) Polyvinyl chloride 50 parts by mass Sodiumstearate  1 part by mass Dioctyl phthalate 20 parts by mass

The materials described above were mixed and dispersed by means of aHenschel mixer and kneaded by means of a pressure kneader and a doubleroll mill, and the kneaded matter was extrusion-molded in the form of alinear body. Then, the molded matter was subjected to heat treatment inthe air in order to remove the residual plasticizer and solidified(dried), and it was then burned at 1000° C. in nitrogen gas atmosphere.Finally, the burned pencil lead was dipped in an α-olefin oligomer(Lipolupe 20, manufactured by Lion Corporation) to obtain a lead HB fora mechanical pencil having a diameter of 0.570 mm.

Comparative Example 24

A nanodiamond-containing burned pencil lead was obtained in the samemanner as in Example 19, except that nanoparticles A: diamondnanoparticles (specific surface area: 208 m²/g, my value: 50 nm,manufactured by Sumiseki Materials Co., Ltd.) was changed to the sameamount of nanoparticles B: diamond nanoparticles (specific surface area:18.54 m²/g, my value: 100 nm, manufactured by Sumiseki Materials Co.,Ltd.).

Comparative Example 25

A nanodiamond-containing burned pencil lead was obtained in the samemanner as in Example 19, except that nanoparticles A: diamondnanoparticles (specific surface area: 208 m²/g, my value: 50 nm,manufactured by Sumiseki Materials Co., Ltd.) was changed to the sameamount of nanoparticles B: diamond nanoparticles (cluster diamond,specific surface area: 820 m²/g, my value: 20 nm).

Comparative Example 26

A nanodiamond-containing burned pencil lead was obtained in the samemanner as in Example 19, except that flake natural graphite A (ab facehaving a flatness of 0.2 μm, my value: 8 μm, thickness of a c axis: 1μm, aspect ratio: 8) used in Example 19 described above was changed tothe same amount of flake natural graphite B (ab face having a flatnessof 3 μm, my value: 10 μm, thickness of a c axis: 1 μm, aspect ratio:10).

Comparative Example 27

A nanodiamond-containing burned pencil lead was obtained in the samemanner as in Example 19, except that flake natural graphite A (ab facehaving a flatness of 0.2 μm, my value: 8 μm, thickness of a c axis: 1μm, aspect ratio: 8) used in Example 19 described above was changed tothe same amount of flake natural graphite C (ab face having a flatnessof 0.2 μm, my value: 3 μm, thickness of a c axis: 1 μm, aspect ratio:3).

Comparative Example 28

The pencil lead body obtained in the same composition as in Example 19described above except containing no nanoparticles A was impregnatedunder pressure, as was the case with Example 19, with liquid A used inExample 19, and a nanodiamond-non-containing burned pencil lead wasobtained.

Comparative Example 29 Based on Example 11 of Japanese PatentApplication Laid-Open No. 138031/2007

Flake natural graphite (ab face having a flatness of 69 parts by mass 3μm, mv value: 10 μm, thickness of a c axis: 1 μm, an aspect ratio: 10)Diamond nanoparticles (cluster diamond, specific  1 part by mass surfacearea: 820 m²/g, mv value: 5 nm) Kaolinite clay 15 parts by massHalloysite clay 15 parts by mass Water 30 parts by mass

The materials described above were mixed and dispersed by means of aHenschel mixer, and the mixture was sufficiently kneaded with heating bymeans of a double roll mill until a moisture there was reduced to about18 parts by mass. The kneaded matter obtained was extrusion-molded inthe form of a linear body by means of a dice for extrusion, and then themolded matter was subjected to heat treatment at 120° C. for 20 hours inthe air to remove residual moisture and burned up to 1200° C. in 10hours and at 1200° C. for 1 hour in nitrogen gas atmosphere.

Next, the burned pencil lead body was impregnated with liquid F (MiyoshiControlled Lard) used in Example 6 to obtain a wood case pencil leadhaving a diameter of 2.05 mm.

Comparative Example 30

The pencil lead body obtained in the same composition as in Example 24described above except containing no nanoparticles A was impregnated, aswas the case with Example 24, with liquid F (Miyoshi Controlled Lard)used in Example 24, and a wood case pencil lead having a diameter of2.05 mm was obtained.

The respective burned pencil leads (pencil leads for a mechanical penciland wood case pencil leads) obtained in Examples 19 to 24 andComparative Examples 23 to 30 were used to evaluate bending strength,compression strength (N), an abrasion amount (mm), intensity, an erasingrate (%), a frictional coefficient (static and dynamic), a nanoparticlenumber, writing feeling by sensory evaluation, difficulty of stain andinitial sliding by the respective evaluation methods described above.The results thereof are shown in the following Table 4.

TABLE 4 Lead Break- Bending Com- Frictional Nano- Sensory evaluationdiameter ing strength pression Abrasion Erasing coefficient particleWriting Difficulty Initial mm load N MPa strength N mm Intensity rate %Static Dynamic number feeling of stain sliding Example 19 0.565 0.757427.5 26.17 2.744 0.516 98.25 0.524 0.179 17 ⊚ ⊚ ⊚ Example 20 0.5650.754 425.8 26.15 2.112 0.399 95.17 0.622 0.199 17 ⊚ ⊚ ◯ Example 210.565 0.755 426.4 26.15 2.632 0.444 98.01 0.546 0.188 17 ⊚ ◯ ⊚ Example22 0.565 0.755 426.4 26.15 2.863 0.495 97.09 0.597 0.181 17 ⊚ ⊚ ◯Example 23 0.565 0.753 425.3 26.16 2.218 0.487 94.67 0.500 0.198 17 ◯ ◯⊚ Example 24 2.050 6.600 117.1 — 1.526 0.412 97.40 0.686 0.198 11 ⊚ ⊚ ⊚Comparative 0.570 0.751 413.1 26.01 1.782 0.432 95.51 0.750 0.200 1000or ⊚ ⊚ Δ Example 23 more Comparative 0.570 0.694 381.7 26.48 3.089 0.48194.11 0.746 0.223 17 ◯ ⊚ Δ Example 24 Comparative 0.570 0.658 361.922.35 1.642 0.371 88.68 0.778 0.231 102 Δ Δ Δ Example 25 Comparative0.570 0.746 410.3 25.78 2.711 0.448 92.36 0.777 0.225 17 ◯ Δ Δ Example26 Comparative 0.570 0.721 396.6 26.24 3.720 0.498 89.26 0.803 0.211 17◯ X X Example 27 Comparative 0.570 0.750 412.5 26.15 2.447 0.485 97.430.759 0.221 0 Δ Δ Δ Example 28 Comparative 2.050 5.990 106.2 — 1.6000.411 94.69 0.706 0.219 1000 or ◯ ◯ ⊚ Example 29 more Comparative 2.0506.630 117.6 — 1.508 0.399 96.85 0.770 0.229 0 Δ Δ Δ Example 30

Comments on Examples and Comparative Examples in the Fourth Embodiment

As apparent from the results shown in Table 4, it has been found thatthe respective pencil leads for a mechanical pencil obtained in Examples19 to 23 and the wood case pencil lead obtained in Example 24 eachfalling in the scope of the present invention are excellent in bendingstrength and compression strength and have a satisfactory color formingproperty and satisfactory drawn line intensity and that they have lessabrasion, a good erasing property, good initial sliding and good writingfeeling (pen touch) and result in being difficult of stain as comparedwith the respective pencil leads for a mechanical pencil obtained inComparative Examples 23 to 28 and the wood case pencil leads obtained inComparative Examples 29 and 30 each falling outside the scope of thepresent invention.

In contrast with this, to observe individually the comparative examples,Comparative Example 23 is based on Example 11 of Japanese PatentApplication Laid-Open No. 138031/2007; Comparative Examples 24 and 25are cases where the nanoparticles falling outside the scope of thepresent invention were used; Comparative Examples 26 and 21 are caseswhere the flake graphites falling outside the scope of the presentinvention were used; Comparative Example 28 is a case where the pencillead was produced without adhering nanoparticles A in blending thematerials in Example 1 described above; and it has been found that thetargeted pencil leads of the present invention are not obtained from theabove pencil leads. Further, Comparative Example 29 provides the woodcase pencil lead based on Example 11 of Japanese Patent ApplicationLaid-Open No. 138031/2007; Comparative Example 30 provides the wood casepencil lead in which the nanoparticles were not used; and it has beenfound that the targeted pencil leads are not obtained from the abovewood case pencil leads.

Examples 25 to 31 and Comparative Examples 31 to 37 Fifth EmbodimentExample 25

Flake natural graphite A (ab face having a flatness 40 parts by mass of0.2 μm, mv value: 8 μm, thickness of a c axis: 1 μm, aspect ratio: 8)Nanoparticles A: diamond nanoparticles (sphericity: 0.4 part by mass 10nm, mv value: 50 nm, manufactured by Sumiseki Materials Co., Ltd.)Polyvinyl chloride 40 parts by mass Sodium stearate 1 part by massDioctyl phthalate 19 parts by mass

First, the nanoparticles and dioctyl phthalate were dispersed for 180minutes by means of a bead mill, and the other materials described abovewere mixed and dispersed (mixing and dispersing time: 20 minutes,hereinafter the same shall apply) by means of a Henschel mixer. Themixture was kneaded by means of a pressure kneader and a roll mill, andafter molded, the molded matter was dried to remove dioctyl phthalateand subjected to burning treatment at 1000° C. for 10 hours in nitrogengas atmosphere, whereby a burned pencil lead body having a diameter of0.565 mm and a length of 60 mm was produced.

Next, the burned pencil lead body described above was impregnated(impregnation time: 180 minutes, hereinafter the same shall apply) at apressure of 1 MPa with liquid A (liquid temperature: 100° C.,hereinafter the same shall apply) described below, and ananodiamond-containing burned pencil lead was obtained.

Liquid A: dimethyl silicone oil KF 96-30CS (dynamic viscosity: 30 mm²/s,refractive index: 1.401, manufactured by Shin-Etsu Chemical Co., Ltd.)

Nanoparticles A described above had a my value of 0.125 based on 100 ofa volume average diameter (mv value) of flake natural graphite A.

Example 26

Liquid B: CMC-Na 1 wt % in distilled water (dynamic viscosity: 7 mm²/s,refractive index: 1.345)

The burned pencil lead body obtained in Example 25 was impregnated at apressure of 1 MPa with liquid B described above to obtain ananodiamond-containing burned pencil lead.

Example 27

Liquid C: trimethylpentaphenyltrisiloxane (dynamic viscosity: 175 mm²/s,refractive index: 1.580, manufactured by Toray Industries, Inc.)

The burned pencil lead body obtained in Example 25 was impregnated at apressure of 1 MPa with liquid C described above to obtain ananodiamond-containing burned pencil lead.

Example 28

Liquid D: dimethyl silicone KF 96L-5cs (dynamic viscosity: 5 mm²/s,refractive index: 1.396, manufactured by Shin-Etsu Chemical Co., Ltd.)

The burned pencil lead body obtained in Example 25 was impregnated at apressure of 1 MPa with liquid D described above to obtain ananodiamond-containing burned pencil lead.

Example 29

Liquid E: dimethyl silicone KF 96-500cs (dynamic viscosity: 500 mm²/s,refractive index: 1.403, manufactured by Shin-Etsu Chemical Co., Ltd.)

The burned pencil lead body obtained in Example 25 was impregnated at apressure of 1 MPa with liquid E described above to obtain ananodiamond-containing burned pencil lead.

Example 30

The burned pencil lead body obtained in Example 25 described above wasimpregnated at a pressure of 1 MPa with liquid F described below toobtain a nanodiamond-containing burned pencil lead.

Liquid F: obtained by dispersing 0.1 part by mass of nanoparticles B(sphericity: 1 nm, my value: 5 nm, manufactured by Sumiseki MaterialsCo., Ltd.) in liquid A.

Example 31

Flake natural graphite A (ab face having a flatness 70 parts by mass of0.2 μm, mv value: 8 μm, thickness of a c axis: 1 μm, aspect ratio: 8)Nanoparticles A: diamond nanoparticles (sphericity: 0.4 part by mass 10nm, mv value: 50 nm, manufactured by Sumiseki Materials Co., Ltd.)Kaolinite clay 15 parts by mass Halloysite clay 15 parts by mass Water30 parts by mass

First, the nanoparticles and water were dispersed for 180 minutes bymeans of a bead mill, and the other materials described above were mixedand dispersed by means of a Henschel mixer. The mixture was sufficientlykneaded with heating by means of a double roll mill until a moisturethere was reduced to about 18 parts by mass. The kneaded matter obtainedwas extrusion-molded in the form of a linear body by means of a dice forextrusion, and then the molded matter was subjected to heat treatment at120° C. for 20 hours in the air to remove residual moisture and burnedup to 1200° C. in 10 hours and at 1200° C. for 1 hour in nitrogen gasatmosphere.

Next, the burned pencil lead body was impregnated with liquid Gdescribed below to obtain a wood case pencil lead having a diameter of2.05 mm.

Liquid G: Miyoshi Controlled Lard (manufactured by Miyoshi Oil & FatCo., Ltd.)

Nanoparticles A described above had a my value of 0.125 based on 100 ofa volume average diameter (mv value) of flake natural graphite A.

Comparative Example 31 Based on Example 11 of Japanese PatentApplication Laid-Open No. 138031/2007

Flake natural graphite (flatness of 3 μm, mv value: 49 parts by mass 10μm, thickness of a c axis: 1 μm, an aspect ratio: 10) Diamondnanoparticles (single crystal diamond,  1 part by mass sphericity: 1.5nm, mv value: 5 nm) Polyvinyl chloride 50 parts by mass Sodium stearate 1 part by mass Dioctyl phthalate 20 parts by mass

The materials described above were mixed and dispersed by means of aHenschel mixer and kneaded by means of a pressure kneader and a doubleroll mill, and the kneaded matter was extrusion-molded in the form of alinear body. Then, the molded matter was subjected to heat treatment inthe air in order to remove the residual plasticizer and solidified(dried), and it was then burned at 1000° C. in nitrogen gas atmosphere.Finally, the burned pencil lead was dipped in an α-olefin oligomer(Lipolupe 20, manufactured by Lion Corporation) to obtain a lead HB fora mechanical pencil having a diameter of 0.570 mm.

Comparative Example 32

A nanodiamond-containing burned pencil lead was obtained in the samemanner as in Example 25, except that nanoparticles A: diamondnanoparticles (sphericity: 10 nm, my value: 50 nm, manufactured bySumiseki Materials Co., Ltd.) used in Example 25 described above waschanged to the same amount of nanoparticles B: diamond nanoparticles(sphericity: 25 nm, my value: 50 nm, manufactured by Sumiseki MaterialsCo., Ltd.).

Comparative Example 33

A nanodiamond-containing burned pencil lead was obtained in the samemanner as in Example 25, except that flake natural graphite A (ab facehaving a flatness of 0.2 μm, my value: 8 μm, thickness of a c axis: 1μm, aspect ratio: 8) used in Example 25 described above was changed tothe same amount of flake natural graphite B (ab face having a flatnessof 3 μm, my value: 10 μm, thickness of a c axis: 1 μm, aspect ratio:10).

Comparative Example 34

A nanodiamond-containing burned pencil lead was obtained in the samemanner as in Example 25, except that flake natural graphite A (ab facehaving a flatness of 0.2 μm, my value: 8 μm, thickness of a c axis: 1μm, aspect ratio: 8) used in Example 25 described above was changed tothe same amount of flake natural graphite C (ab face having a flatnessof 0.2 μm, my value: 3 μm, thickness of a c axis: 1 μm, aspect ratio:3).

Comparative Example 35

The pencil lead body obtained without dispersing the nanoparticles inExample 25 described above was impregnated under pressure, as was thecase with Example 25, with liquid A used in Example 25 containing nonanoparticles A, and a nanodiamond-non-containing burned pencil lead wasobtained.

Comparative Example 36 Based on Example 11 of Japanese PatentApplication Laid-Open No. 138031/2007

Flake natural graphite (ab face having a flatness 69 parts by mass of 3μm, mv value: 10 μm, thickness of a c axis: 1 μm, aspect ratio: 10)Diamond nanoparticles (single crystal diamond,  1 part by masssphericity: 1.5 nm, mv value: 5 nm) Kaolinite clay 15 parts by massHalloysite clay 15 parts by mass Water 30 parts by mass

The materials described above were mixed and dispersed by means of aHenschel mixer, and the mixture was sufficiently kneaded with heating bymeans of a double roll mill until a moisture there was reduced to about18 parts by mass. The kneaded matter obtained was extrusion-molded inthe form of a linear body by means of a dice for extrusion, and then themolded matter was subjected to heat treatment at 120° C. for 20 hours inthe air to remove residual moisture and burned up to 1200° C. in 10hours and at 1200° C. for 1 hour in nitrogen gas atmosphere.

Next, the burned pencil lead body was impregnated with liquid G (MiyoshiControlled Lard) used in Example 7 to obtain a wood case pencil leadhaving a diameter of 2.05 mm.

Comparative Example 37

A pencil lead body obtained in the same composition as in Example 31described above except containing no nanoparticles A was impregnated, aswas the case with Example 31, with liquid G (Miyoshi Controlled Lard)used in Example 7 to obtain a wood case pencil lead having a diameter of2.05 mm.

The respective burned pencil leads (pencil leads for a mechanical penciland wood case pencil leads) obtained in Examples 25 to 31 andComparative Examples 31 to 37 were used to evaluate bending strength,compression strength (N), an abrasion amount (mm), intensity, an erasingrate (%), a frictional coefficient (static and dynamic), a nanoparticlenumber, writing feeling by sensory evaluation, difficulty of stain andinitial sliding by the respective evaluation methods described above.The results thereof are shown in the following Table 5.

TABLE 5 Lead Break- Bending Com- Frictional Nano- Sensory evaluationdiameter ing strength pression Abrasion Erasing coefficient particleWriting Difficulty Initial mm load N MPa strength N mm Intensity rate %Static Dynamic number feeling of stain sliding Example 25 0.565 0.755426.4 26.15 2.495 0.465 98.12 0.510 0.179 17 ⊚ ⊚ ⊚ Example 26 0.5650.755 426.4 26.15 1.925 0.358 95.20 0.618 0.199 17 ⊚ ⊚ ◯ Example 270.565 0.755 426.4 26.15 2.412 0.401 98.01 0.554 0.188 17 ⊚ ◯ ⊚ Example28 0.565 0.755 426.4 26.15 2.591 0.401 97.08 0.605 0.181 17 ⊚ ⊚ ◯Example 29 0.565 0.755 426.4 26.15 2.015 0.445 94.55 0.515 0.198 17 ◯ ◯⊚ Example 30 0.565 0.735 415.1 25.12 2.011 0.448 95.68 0.681 0.191 58 ⊚⊚ ◯ Example 31 2.050 6.620 117.4 — 1.558 0.423 98.37 0.666 0.189 10 ⊚ ⊚⊚ Comparative 0.570 0.751 413.1 26.01 1.782 0.432 95.51 0.750 0.200 1000or ⊚ ⊚ Δ Example 31 more Comparative 0.570 0.755 415.3 26.01 1.782 0.43295.51 0.781 0.202 18 ◯ ⊚ X Example 32 Comparative 0.570 0.745 409.825.85 2.515 0.448 92.23 0.748 0.225 17 ◯ Δ Δ Example 33 Comparative0.570 0.725 398.8 26.23 3.381 0.452 89.20 0.812 0.211 17 ◯ X X Example34 Comparative 0.570 0.755 415.3 26.15 2.215 0.445 97.15 0.768 0.221 0 ΔΔ Δ Example 35 Comparative 2.050 6.350 112.6 — 1.604 0.419 95.92 0.7540.217 1000 or ◯ ◯ ⊚ Example 36 more Comparative 2.050 6.630 117.6 —1.508 0.399 96.85 0.770 0.229 0 Δ Δ Δ Example 37

Comments on Examples and Comparative Examples in the Fifth Embodiment

As apparent from the results shown in Table 5, it has been found thatthe respective pencil leads for a mechanical pencil obtained in Examples25 to 30 and the wood case pencil lead obtained in Example 31 eachfalling in the scope of the present invention are excellent in bendingstrength and compression strength and have a satisfactory color formingproperty and satisfactory drawn line intensity and that they have lessabrasion, a good erasing property, good initial sliding and good writingfeeling (pen touch) and result in being difficult of stain as comparedwith the respective pencil leads for a mechanical pencil obtained inComparative Examples 31 to 35 and the wood case pencil leads obtained inComparative Examples 36 and 37 each falling outside the scope of thepresent invention.

In contrast with this, to observe individually the comparative examples,Comparative Example 31 is based on Example 11 of Japanese PatentApplication Laid-Open No. 138031/2007; Comparative Example 32 is a casewhere the nanoparticles falling outside the scope of the presentinvention were used; Comparative Examples 33 and 34 are cases where theflake graphites falling outside the scope of the present invention wereused; Comparative Example 35 is a case where the pencil lead obtainedwithout dispersing the nanoparticles in blending the materials inExample 25 described above was produced; and it has been found that thetargeted pencil leads of the present invention which exert stronglylubrication action are not obtained from the above pencil leads.Further, Comparative Example 36 provides the wood case pencil lead basedon Example 11 of Japanese Patent Application Laid-Open No. 138031/2007;Comparative Example 37 provides the wood case pencil lead in which thenanoparticles were not used; and it has been found that the targetedpencil leads are not obtained from the above wood case pencil leads.

Examples 32 to 38 and Comparative Examples 38 to 45 Sixth EmbodimentExample 32

Flake natural graphite A (ab face having a flatness 40 parts by mass of0.2 μm, mv value: 8 μm, thickness of a c axis: 1 μm, aspect ratio: 8)Nanoparticles A: diamond nanoparticles (specific 0.4 part by masssurface area: 208 m²/g, mv value: 50 nm, manufactured by SumisekiMaterials Co., Ltd.) Polyvinyl chloride 40 parts by mass Sodium stearate1 part by mass Dioctyl phthalate 19 parts by mass

First, the nanoparticles and dioctyl phthalate were dispersed for 180minutes by means of a bead mill, and the other materials described abovewere mixed and dispersed (mixing and dispersing time: 20 minutes,hereinafter the same shall apply) by means of a Henschel mixer. Themixture was kneaded by means of a pressure kneader and a roll mill, andafter molded, the molded matter was dried to remove dioctyl phthalateand then subjected to burning treatment at 1000° C. for 10 hours innitrogen gas atmosphere, whereby a burned pencil lead body having adiameter of 0.565 mm and a length of 60 mm was produced.

Next, the burned pencil lead body described above was impregnated(impregnation time: 180 minutes, hereinafter the same shall apply) at apressure of 1 MPa with liquid A (liquid temperature: 100° C.,hereinafter the same shall apply) described below, and ananodiamond-containing burned pencil lead was obtained.

Liquid A: dimethyl silicone oil KF 96-30CS (dynamic viscosity: 30 mm²/s,refractive index: 1.401, manufactured by Shin-Etsu Chemical Co., Ltd.)

Nanoparticles A described above had a my value of 0.125 based on 100 ofa volume average diameter (mv value) of flake natural graphite A.

Example 33

Liquid B: CMC-Na 1 wt % in distilled water (dynamic viscosity: 7 mm²/s,refractive index: 1.345)

The burned pencil lead body obtained in Example 32 was impregnated at apressure of 1 MPa with liquid B described above to obtain ananodiamond-containing burned pencil lead.

Example 34

Liquid C: trimethylpentaphenyltrisiloxane (dynamic viscosity: 175 mm²/s,refractive index: 1.580, manufactured by Toray Industries, Inc.)

The burned pencil lead body obtained in Example 32 was impregnated at apressure of 1 MPa with liquid C described above to obtain ananodiamond-containing burned pencil lead.

Example 35

Liquid D: dimethyl silicone KF 96L-5cs (dynamic viscosity: 5 mm²/s,refractive index: 1.396, manufactured by Shin-Etsu Chemical Co., Ltd.)

The burned pencil lead body obtained in Example 32 was impregnated at apressure of 1 MPa with liquid D described above to obtain ananodiamond-containing burned pencil lead.

Example 36

Liquid E: dimethyl silicone KF 96-500cs (dynamic viscosity: 500 mm²/s,refractive index: 1.403, manufactured by Shin-Etsu Chemical Co., Ltd.)

The burned pencil lead body obtained in Example 32 was impregnated at apressure of 1 MPa with liquid E described above to obtain ananodiamond-containing burned pencil lead.

Example 37

The burned pencil lead body obtained in Example 32 was impregnated at apressure of 1 MPa with liquid F described below to obtain ananodiamond-containing burned pencil lead. Liquid F: obtained bydispersing 0.1 part by mass of the nanoparticles B (sphericity: 1 nm, myvalue: 5 nm, manufactured by Sumiseki Materials Co., Ltd.) in liquid A.

Example 38

Flake natural graphite A (ab face having a flatness 70 parts by mass of0.2 μm, mv value: 8 μm, thickness of a c axis: 1 μm, aspect ratio: 8)Nanoparticles A: diamond nanoparticles (specific 0.4 part by masssurface area: 208 m²/g, mv value: 50 nm, manufactured by SumisekiMaterials Co., Ltd.) Kaolinite clay 15 parts by mass Halloysite clay 15parts by mass Water 30 parts by mass

First, the nanoparticles and water were dispersed for 180 minutes bymeans of a bead mill, and the other materials described above were mixedand dispersed by means of a Henschel mixer. The mixture was sufficientlykneaded with heating by means of a double roll mill until a moisturethere was reduced to about 18 parts by mass. The kneaded matter obtainedwas extrusion-molded in the form of a linear body by means of a dice forextrusion, and then the molded matter was subjected to heat treatment at120° C. for 20 hours in the air to remove residual moisture and burnedup to 1200° C. in 10 hours and at 1200° C. for 1 hour in nitrogen gasatmosphere.

Next, the burned pencil lead body was impregnated with liquid Gdescribed below to obtain a wood case pencil lead having a diameter of2.05 mm.

Liquid G: Miyoshi Controlled Lard (manufactured by Miyoshi Oil & FatCo., Ltd.)

The nanoparticles described above had a my value of 0.125 based on 100of a volume average diameter (mv value) of flake natural graphite A.

Comparative Example 38 Based on Example 11 of Japanese PatentApplication Laid-Open No. 138031/2007

Flake natural graphite having a flatness of 3 μm, 49 parts by mass mvvalue: 10 μm, thickness of a c axis: 1 μm, aspect ratio of 10) Diamondnanoparticles (cluster diamond, specific  1 part by mass surface area:820 m²/g, mv value: 5 nm) Polyvinyl chloride 50 parts by mass Sodiumstearate  1 part by mass Dioctyl phthalate 20 parts by mass

The materials described above were mixed and dispersed by means of aHenschel mixer and kneaded by means of a pressure kneader and a doubleroll mill, and the kneaded matter was extrusion-molded in the form of alinear body. Then, the molded matter was subjected to heat treatment inthe air in order to remove the residual plasticizer and solidified(dried), and it was then burned at 1000° C. in nitrogen gas atmosphere.Finally, the burned pencil lead was dipped in an α-olefin oligomer(Lipolupe 20, manufactured by Lion Corporation) to obtain a lead HB fora mechanical pencil having a diameter of 0.570 mm.

Comparative Example 39

A nanodiamond-containing burned pencil lead was obtained in the samemanner as in Example 32, except that the nanoparticles A: diamondnanoparticles (specific surface area: 208 m²/g, my value: 50 nm,manufactured by Sumiseki Materials Co., Ltd.) used in Example 32described above was changed to the same amount of nanoparticles B:diamond nanoparticles (specific surface area: 18.54 m²/g, my value: 100nm, manufactured by Sumiseki Materials Co., Ltd.).

Comparative Example 40

A nanodiamond-containing burned pencil lead was obtained in the samemanner as in Example 32, except that the nanoparticles A: diamondnanoparticles (specific surface area: 208 m²/g, my value: 50 nm,manufactured by Sumiseki Materials Co., Ltd.) used in Example 32described above was changed to the same amount of the nanoparticles C:cluster diamond nanoparticles (specific surface area: 848 m²/g, myvalue: 20 nm, manufactured by Sumiseki Materials Co., Ltd.).

Comparative Example 41

A nanodiamond-containing burned pencil lead was obtained in the samemanner as in Example 32, except that flake natural graphite A (ab facehaving a flatness of 0.2 μm, my value: 8 μm, thickness of a c axis: 1μm, aspect ratio: 8) used in Example 32 described above was changed tothe same amount of flake natural graphite B (ab face having a flatnessof 3 μm, my value: 10 μm, thickness of a c axis: 1 μm, aspect ratio:10).

Comparative Example 42

A nanodiamond-containing burned pencil lead was obtained in the samemanner as in Example 32, except that flake natural graphite A (ab facehaving a flatness of 0.2 μm, my value: 8 μm, thickness of a c axis: 1μm, aspect ratio: 8) used in Example 32 described above was changed tothe same amount of flake natural graphite C (ab face having a flatnessof 0.2 μm, my value: 3 μm, thickness of a c axis: 1 μm, aspect ratio:3).

Comparative Example 43

The pencil lead body obtained without adding nanoparticles A in blendingthe materials in Example 32 described above was impregnated underpressure, as was the case with Example 1, with liquid A used in Example32 containing no nanoparticles A, and a nanodiamond-non-containingburned pencil lead was obtained.

Comparative Example 44 Based on Example 11 of Japanese PatentApplication Laid-Open No. 138031/2007

Flake natural graphite (ab face having a flatness: 69 parts by mass 3μm, mv value: 10 μm, thickness of a c axis: 1 μm, aspect ratio: 10)Diamond nanoparticles (cluster diamond, specific  1 part by mass surfacearea: 820 m²/g, mv value: 5 nm) Kaolinite clay 15 parts by massHalloysite clay 15 parts by mass Water 30 parts by mass

The materials described above were mixed and dispersed by means of aHenschel mixer, and the mixture was sufficiently kneaded with heating bymeans of a double roll mill until a moisture there was reduced to about18 parts by mass. The kneaded matter obtained was extrusion-molded inthe form of a linear body by means of a dice for extrusion, and then themolded matter was subjected to heat treatment at 120° C. for 20 hours inthe air to remove residual moisture and burned up to 1200° C. in 10hours and at 1200° C. for 1 hour in nitrogen gas atmosphere.

Next, the burned pencil lead body was impregnated with liquid G (MiyoshiControlled Lard) used in Example 38 to obtain a wood case pencil leadhaving a diameter of 2.05 mm.

Comparative Example 45

A pencil lead body obtained in the same composition as in Example 38described above except containing no nanoparticles A was impregnated, aswas the case with Example 7, with liquid G (Miyoshi Controlled Lard)used in Example 7 to obtain a wood case pencil lead having a diameter of2.05 mm.

The respective burned pencil leads (pencil leads for a mechanical penciland wood case pencil leads) obtained in Examples 32 to 38 andComparative Examples 38 to 45 were used to evaluate bending strength,compression strength (N), an abrasion amount (mm), intensity, an erasingrate (%), a frictional coefficient (static and dynamic), a nanoparticlenumber, writing feeling by sensory evaluation, difficulty of stain andinitial sliding by the respective evaluation methods described above.The results thereof are shown in the following Table 6.

TABLE 6 Lead Break- Bending Com- Frictional Nano- Sensory evaluationdiameter ing strength pression Abrasion Erasing coefficient particleWriting Difficulty Initial mm load N MPa strength N mm Intensity rate %Static Dynamic number feeling of stain sliding Example 32 0.565 0.755426.4 26.15 2.495 0.465 98.12 0.510 0.179 17 ⊚ ⊚ ⊚ Example 33 0.5650.755 426.4 26.15 1.925 0.358 95.20 0.618 0.199 17 ⊚ ⊚ ◯ Example 340.565 0.755 426.4 26.15 2.412 0.401 98.01 0.554 0.188 17 ⊚ ◯ ⊚ Example35 0.565 0.755 426.4 26.15 2.591 0.401 97.08 0.605 0.181 17 ⊚ ⊚ ◯Example 36 0.565 0.755 426.4 26.15 2.015 0.445 94.55 0.515 0.198 17 ◯ ◯⊚ Example 37 0.565 0.735 415.1 25.12 2.011 0.448 95.68 0.681 0.191 58 ⊚⊚ ◯ Example 38 2.050 6.580 116.7 — 1.562 0.425 98.74 0.617 0.188 11 ⊚ ⊚⊚ Comparative 0.570 0.751 413.1 26.01 1.782 0.432 95.51 0.750 0.200 1000or ⊚ ⊚ Δ Example 38 more Comparative 0.570 0.691 380.1 26.51 2.815 0.43994.15 0.761 0.223 2 ◯ ⊚ Δ Example 39 Comparative 0.570 0.653 359.2 22.151.512 0.331 88.75 0.784 0.231 52 Δ Δ Δ Example 40 Comparative 0.5700.745 409.8 25.85 2.515 0.448 92.23 0.784 0.225 17 ◯ Δ Δ Example 41Comparative 0.570 0.725 398.8 26.23 3.381 0.452 89.20 0.812 0.211 17 ◯ XX Example 42 Comparative 0.570 0.755 415.3 26.15 2.215 0.445 97.15 0.7680.221 0 Δ Δ Δ Example 43 Comparative 2.050 5.990 106.2 — 1.600 0.41194.69 0.706 0.219 1000 or ◯ ◯ ⊚ Example 44 more Comparative 2.050 6.630117.6 — 1.508 0.399 96.85 0.770 0.229 0 Δ Δ Δ Example 45

Comments on Examples and Comparative Examples in the Sixth Embodiment

As apparent from the results shown in Table 6, it has been found thatthe respective pencil leads for a mechanical pencil obtained in Examples32 to 37 and the wood case pencil lead obtained in Example 38 eachfalling in the scope of the present invention are excellent in bendingstrength and compression strength and have a satisfactory color formingproperty and satisfactory drawn line intensity and that they have lessabrasion, a good erasing property, good initial sliding and good writingfeeling (pen touch) and result in being difficult of stain as comparedwith the respective pencil leads for a mechanical pencil obtained inComparative Examples 38 to 43 and the wood case pencil leads obtained inComparative Examples 44 and 45 each falling outside the scope of thepresent invention.

In contrast with this, to observe individually the comparative examples,Comparative Example 38 is based on Example 11 of Japanese PatentApplication Laid-Open No. 138031/2007; Comparative Examples 39 and 40are cases where the nanoparticles falling outside the scope of thepresent invention were used; Comparative Examples 41 and 42 are caseswhere flake graphites falling outside the scope of the present inventionwere used; Comparative Example 43 is a case where the pencil leadobtained without dispersing the nanoparticles in blending the materialsin Example 32 described above was produced; and it has been found thatthe targeted pencil leads of the present invention which exert stronglylubrication action are not obtained from the above pencil leads.Further, Comparative Example 44 provides the wood case pencil lead basedon Example 11 of Japanese Patent Application Laid-Open No. 138031/2007;Comparative Example 45 provides the wood case pencil lead in which thenanoparticles were not used; and it has been found that the targetedpencil leads are not obtained from the above wood case pencil leads.

INDUSTRIAL APPLICABILITY

Obtained is a production process for a pencil lead which has better andsmoother writing feeling and which provides drawn lines with highintensity and a brilliant black color, wherein it is a pencil lead usedfor a mechanical pencil of a type in which a lead is rotated wheneverwritten to always carry out writing by a fresh part in addition topencil leads used for mechanical pencils, wood case holder pencils andthe like.

1-21. (canceled)
 22. A pencil lead containing flake graphite which hasan ab face having a flatness of at least 2 μm or less and in which anaspect ratio of an a axis or a b axis in the ab face to a c axis is 5 ormore, wherein nanoparticles having a my value of 0.05 to 2 based on 100of a volume average diameter (my value) of the graphite and having asphericity of 0.1 to 20 nm are brought into contact with the ab face ofthe graphite.
 23. A pencil lead containing flake graphite which has anab face having a flatness of at least 2 μm or less and in which anaspect ratio of an a axis or a b axis in the ab face to a c axis is 5 ormore, wherein nanoparticles having a my value of 0.05 to 2 based on 100of a volume average diameter (my value) of the graphite and having aspecific surface area of 50 to 800 m²/g are brought into contact withthe ab face of the graphite.
 24. A pencil lead containing flake graphitewhich has an ab face having a flatness of at least 2 μm or less and inwhich an aspect ratio of an a axis or a b axis in the ab face to a caxis is 5 or more, wherein nanoparticles having a my value of 0.05 to 2based on 100 of a volume average diameter (my value) of the graphite andhaving a sphericity of 0.1 to 20 nm are adhered to the ab face of thegraphite.
 25. A pencil lead containing flake graphite which has an abface having a flatness of at least 2 μm or less and in which an aspectratio of an a axis or a b axis in the ab face to a c axis is 5 or more,wherein nanoparticles having a my value of 0.05 to 2 based on 100 of avolume average diameter (my value) of the graphite and having a specificsurface area of 50 to 800 m²/g are adhered to the ab face of thegraphite.
 26. The pencil lead as described in claim 22, wherein thenanoparticles used for the pencil lead are carbon nanoparticles.
 27. Thepencil lead as described in claim 23, wherein the nanoparticles used forthe pencil lead are carbon nanoparticles.
 28. The pencil lead asdescribed in claim 24, wherein the nanoparticles used for the pencillead are carbon nanoparticles.
 29. The pencil lead as described in claim25, wherein the nanoparticles used for the pencil lead are carbonnanoparticles.
 30. The pencil lead as described in claim 26, wherein thecarbon nanoparticles are diamond.
 31. The pencil lead as described inclaim 27, wherein the carbon nanoparticles are diamond.
 32. The pencillead as described in claim 28, wherein the carbon nanoparticles arediamond.
 33. The pencil lead as described in claim 29, wherein thecarbon nanoparticles are diamond.
 34. The pencil lead as described inclaim 22, wherein the nanoparticles have a volume average diameter (myvalue) of 4 to 100 nm.
 35. The pencil lead as described in claim 23,wherein the nanoparticles have a volume average diameter (my value) of 4to 100 nm.
 36. The pencil lead as described in claim 24, wherein thenanoparticles have a volume average diameter (my value) of 4 to 100 nm.37. The pencil lead as described in claim 25, wherein the nanoparticleshave a volume average diameter (my value) of 4 to 100 nm.
 38. The pencillead as described in claim 24, wherein a clearance formed between theflake graphites by the nanoparticles is impregnated with a liquid havinga refractive index of 1.3 to 1.5 and a viscosity of 7 to 200 mm²/s at25° C.
 39. The pencil lead as described in claim 25, wherein a clearanceformed between the flake graphites by the nanoparticles is impregnatedwith a liquid having a refractive index of 1.3 to 1.5 and a viscosity of7 to 200 mm²/s at 25° C.
 40. The pencil lead as described in claim 22,wherein a total frictional coefficient obtained by dividing an averagevalue (n=10) of total frictional forces in lines drawn by a drawingmethod using a drawing apparatus prescribed in JIS S 6005:2007 by awriting load is 0.191 to 0.218.
 41. The pencil lead as described inclaim 23, wherein a total frictional coefficient obtained by dividing anaverage value (n=10) of total frictional forces in lines drawn by adrawing method using a drawing apparatus prescribed in JIS S 6005:2007by a writing load is 0.191 to 0.218.
 42. The pencil lead as described inclaim 24, wherein a total frictional coefficient obtained by dividing anaverage value (n=10) of total frictional forces in lines drawn by adrawing method using a drawing apparatus prescribed in JIS S 6005:2007by a writing load is 0.191 to 0.218.
 43. The pencil lead as described inclaim 25, wherein a total frictional coefficient obtained by dividing anaverage value (n=10) of total frictional forces in lines drawn by adrawing method using a drawing apparatus prescribed in JIS S 6005:2007by a writing load is 0.191 to 0.218.
 44. The pencil lead as described inclaim 22, wherein when a polished cross section of the pencil lead isobserved in an area of 5 μm×5 μm by means of FE-SEM (acceleratingvoltage: 5 kV), 1 to 300 particles of the nanoparticles are observed.45. The pencil lead as described in claim 23, wherein when a polishedcross section of the pencil lead is observed in an area of 5 μm×5 μm bymeans of FE-SEM (accelerating voltage: 5 kV), 1 to 300 particles of thenanoparticles are observed.
 46. The pencil lead as described in claim24, wherein when a polished cross section of the pencil lead is observedin an area of 5 μm×5 μm by means of FE-SEM (accelerating voltage: 5 kV),1 to 300 particles of the nanoparticles are observed.
 47. The pencillead as described in claim 25, wherein when a polished cross section ofthe pencil lead is observed in an area of 5 μm×5 μm by means of FE-SEM(accelerating voltage: 5 kV), 1 to 300 particles of the nanoparticlesare observed.
 48. A production process for the pencil lead as describedin claim 22, wherein a lead body of a pencil lead containing flakegraphite which has an ab face having a flatness of at least 2 μm or lessand in which an aspect ratio of an a axis or a b axis in the ab face toa c axis is 5 or more is formed; then nanoparticles are dispersed in aliquid having a refractive index of 1.3 to 1.5 and a viscosity of 7 to200 mm²/s at 25° C., and the lead body is then impregnated with theliquid.
 49. A production process for the pencil lead as described inclaim 23, wherein a lead body of a pencil lead containing flake graphitewhich has an ab face having a flatness of at least 2 μm or less and inwhich an aspect ratio of an a axis or a b axis in the ab face to a caxis is 5 or more is formed; then nanoparticles are dispersed in aliquid having a refractive index of 1.3 to 1.5 and a viscosity of 7 to200 mm²/s at 25° C., and the lead body is then impregnated with theliquid.
 50. A production process for the pencil lead as described inclaim 24, wherein flake graphite which has an ab face having a flatnessof at least 2 μm or less and in which an aspect ratio of an a axis or ab axis in the ab face to a c axis is 5 or more is brought into contactwith nanoparticles having a my value of 0.05 to 2 based on 100 of avolume average diameter (my value) of the graphite and having asphericity of 0.1 to 20 nm; then the nanoparticles are fixed thereon toprepare a composite; a lead body is formed by using the compositegraphite and impregnated with a liquid having a refractive index of 1.3to 1.5 and a viscosity of 7 to 200 mm²/s at 25° C.
 51. A productionprocess for the pencil lead as described in claim 25, wherein flakegraphite which has an ab face having a flatness of at least 2 μm or lessand in which an aspect ratio of an a axis or a b axis in the ab face toa c axis is 5 or more is brought into contact with nanoparticles havinga my value of 0.05 to 2 based on 100 of a volume average diameter (myvalue) of the graphite and having a specific surface area of 50 to 800m²/g; then the nanoparticles are fixed thereon to prepare a composite; alead body is formed by using the composite graphite and impregnated witha liquid having a refractive index of 1.3 to 1.5 and a viscosity of 7 to200 mm²/s at 25° C.
 52. The production process for the pencil lead asdescribed in claim 50, wherein the graphite is brought into contact withthe nanoparticles by electrostatic force.
 53. The production process forthe pencil lead as described in claim 51, wherein the graphite isbrought into contact with the nanoparticles by electrostatic force. 54.A production process for a pencil lead, wherein flake graphite which hasan ab face having a flatness of at least 2 μm or less and in which anaspect ratio of an a axis or a b axis in the ab face to a c axis is 5 ormore and nanoparticles having a my value of 0.05 to 2 based on 100 of avolume average diameter (my value) of the graphite and having asphericity of 0.1 to 20 nm are dispersed in a plasticizer or a solventand kneaded, and then a lead body is formed from the kneaded matter. 55.A production process for a pencil lead, wherein flake graphite which hasan ab face having a flatness of at least 2 μm or less and in which anaspect ratio of an a axis or a b axis in the ab face to a c axis is 5 ormore and nanoparticles having a my value of 0.05 to 2 based on 100 of avolume average diameter (my value) of the graphite and having specificsurface area of 50 to 800 m²/g are dispersed in a plasticizer or asolvent and kneaded, and then a lead body is formed from the kneadedmatter.
 56. The production process for a pencil lead as described inclaim 54, wherein the nanoparticles are carbon nanoparticles.
 57. Theproduction process for a pencil lead as described in claim 55, whereinthe nanoparticles are carbon nanoparticles.
 58. The production processfor a pencil lead as described in claim 56, wherein the nanoparticlesare diamond.
 59. The production process for a pencil lead as describedin claim 57, wherein the nanoparticles are diamond.
 60. The productionprocess for a pencil lead as described in claim 54, wherein thenanoparticles have a volume average diameter (my value) of 4 to 100 nm.61. The production process for a pencil lead as described in claim 55,wherein the nanoparticles have a volume average diameter (my value) of 4to 100 nm.
 62. The production process for a pencil lead as described inclaim 54, wherein a total frictional coefficient obtained by dividing anaverage value (n=10) of total frictional forces in lines drawn by adrawing method using a drawing apparatus prescribed in JIS S 6005:2007by a writing load is 0.191 to 0.218.
 63. The production process for apencil lead as described in claim 55, wherein a total frictionalcoefficient obtained by dividing an average value (n=10) of totalfrictional forces in lines drawn by a drawing method using a drawingapparatus prescribed in JIS S 6005:2007 by a writing load is 0.191 to0.218.
 64. The production process for a pencil lead as described inclaim 54, wherein a clearance formed between the flake graphites by thenanoparticles is impregnated with a liquid having a refractive index of1.3 to 1.5 and a viscosity of 7 to 200 mm²/s at 25° C. after forming thelead body.
 65. The production process for a pencil lead as described inclaim 55, wherein a clearance formed between the flake graphites by thenanoparticles is impregnated with a liquid having a refractive index of1.3 to 1.5 and a viscosity of 7 to 200 mm²/s at 25° C. after forming thelead body.
 66. The production process for a pencil lead as described inclaim 54, wherein when a polished cross section of the pencil lead isobserved in an area of 5 μm×5 μm by means of FE-SEM (acceleratingvoltage: 5 kV), 1 to 300 nanoparticles are observed.
 67. The productionprocess for a pencil lead as described in claim 55, wherein when apolished cross section of the pencil lead is observed in an area of 5μm×5 μm by means of FE-SEM (accelerating voltage: 5 kV), 1 to 300nanoparticles are observed.